Ask ten experienced investors "how many stocks should I own?" and you'll get ten different answers, several of them contradictory, and most delivered with total confidence. That's because this isn't really a math question with one right answer — it's a question about how much of your portfolio's fate you're willing to tie to how much of your own research and conviction.
Here's the actual case on both sides, the formula serious position-sizers use to bridge them, and how to land on a number that fits how much work you're actually going to put in.
The Two Camps, in Their Own Words
The diversification camp, built on Harry Markowitz's Modern Portfolio Theory: spread capital across enough uncorrelated positions that no single company's bad news can meaningfully damage the whole portfolio. The academic finding — replicated across decades of research — is that roughly 20-30 stocks across different sectors eliminates the large majority of unsystematic (company-specific) risk. Add a 31st stock and you're barely moving the needle; you've captured almost all the diversification benefit that's available.
The concentration camp, most associated with Warren Buffett: "Diversification is protection against ignorance. It makes little sense if you know what you are doing." Buffett has run Berkshire's public equity portfolio with a handful of positions accounting for the large majority of its value for decades. The logic: if you've done the work to genuinely understand a business, diluting that position to make room for your 25th-best idea just drags down your best idea's contribution to your returns.
Both camps are correct — they're just answering different questions. Diversification theory answers "how do I minimize risk for a given expected return, assuming I have no informational edge on any individual stock?" Concentration theory answers "if I do have a real edge on a small number of ideas, how do I capture the most value from it?" Which camp you belong in depends entirely on which premise is actually true for you.
The 80/20 Reality of Portfolio Returns
One finding shows up again and again in return-attribution studies of both professional and retail portfolios: a small number of positions typically drive the large majority of total gains. Research on long-run stock returns has repeatedly found that a minority of stocks are responsible for the majority of the stock market's total wealth creation over time — most individual stocks underperform, and the aggregate market's return is carried by a relatively small number of exceptional winners.
This cuts both ways for the concentration-vs-diversification question:
- For the diversification case: since you can't reliably know in advance which stocks will be the big winners, owning more names increases your odds of holding at least some of them.
- For the concentration case: if you can identify likely winners with above-average accuracy, holding 30 stocks means your winners are diluted by 29 other positions — most of which are, by definition, not going to be the big winner.
Neither side is wrong about the data. The 80/20 pattern is real. What differs is whether you believe your own stock-picking process is skilled enough to beat the base rate of correctly identifying winners in advance — a belief that should be tested against your own track record, not assumed.
The Kelly Criterion: Sizing Positions to Conviction, Not Equally
If concentration is sometimes right and diversification is sometimes right, the real skill is knowing how much to concentrate on any given idea. The Kelly Criterion, developed originally for bet sizing in gambling and later adopted in quantitative finance, gives a formula for exactly that:
f* = (bp − q) / b
Where f* is the fraction of capital to allocate, p is your estimated probability of the position working out, q is the probability it doesn't (1 − p), and b is the payoff ratio (how much you gain relative to what you risk if you're right).
The output scales naturally with conviction: a position where you have a genuine, well-reasoned edge and a favorable payoff ratio gets a larger allocation than a position you're only lukewarm on. That's the entire point — Kelly sizing formalizes the intuition that not every idea deserves the same-sized bet.
Two practical cautions before using it:
- Full Kelly is too aggressive for real portfolios. The formula assumes you know your true win probability with precision, which no investor actually does. Overestimate your edge even slightly and full Kelly sizing produces brutal drawdowns. Most practitioners who use Kelly sizing at all use fractional Kelly — a quarter to a half of the calculated size — trading some theoretical growth rate for a survivable ride.
- It requires an honest probability estimate, which is the hardest part. The formula is only as good as the
pandbyou feed it. Investors who use Kelly sizing well tend to spend more time interrogating their own confidence than computing the formula itself.
Calibrating Concentration to Your Research Depth
The single most useful practical rule connecting both camps: the number of positions you hold should scale inversely with how much genuine research you're doing per position.
| Research depth per position | Reasonable number of positions | Why |
|---|---|---|
| Deep — read filings, model the business, track quarterly | 8-15 | You can actually stay on top of thesis-invalidating news for this many names |
| Moderate — screener-driven, thesis in a sentence or two, quarterly check-ins | 15-30 | Enough attention to catch major changes, not enough to catch every nuance |
| Light — pattern/technical based, or simply following an index or theme | 30+ or an index fund | Diversification does the risk-reduction work that deep research would otherwise do |
A 12-stock portfolio where you've genuinely done the work on each name is a defensible concentrated strategy. A 12-stock portfolio assembled from headlines and a hot tip is just uncompensated risk wearing a concentration strategy's clothes. The number itself doesn't determine whether you're being prudent — the ratio of conviction-backed-by-research to position size does.
Conviction Tiers: A Simple Framework for Sizing
Rather than sizing every position identically, many disciplined concentrated investors use a tiered system:
- Starter position (1-3% of portfolio) — you like the setup or thesis but want to build conviction with real capital on the line before committing more. Easy to add to, easy to exit without regret.
- Core position (5-10%) — a name you've researched thoroughly and have meaningful, but not maximum, conviction in. The bulk of a typical concentrated portfolio lives here.
- High-conviction position (10-20%+) — reserved for the rare idea where research depth, valuation, and catalyst all line up, and you'd be comfortable explaining to yourself in a year why it deserved an outsized bet.
The tiers only work if they're a decision made in advance, not a label applied after the fact to justify whatever a position happens to have grown into.
The Silent Risk: Concentration You Didn't Choose
The most common way portfolios become dangerously concentrated isn't a deliberate high-conviction bet — it's drift. A position bought at a reasonable 5% allocation triples in value while the rest of the portfolio grows normally, and two years later it's quietly 30% of the account with nobody having made that decision on purpose.
This is the scenario where diversification theory and concentration theory actually agree: an unmanaged concentrated position is the worst of both worlds — you get the company-specific risk of concentration without having deliberately underwritten the size of the bet. The fix isn't necessarily to trim it (a winning thesis that's still intact is still a winning thesis) — it's to notice it's happened and make an active decision, rather than let sizing be decided by price action alone.
This is where automated monitoring earns its keep. A price alert set specifically on your largest positions — separate from general watchlist alerts — flags a big move the moment it happens instead of weeks later during an occasional portfolio review, so concentration by accident becomes concentration by decision.
Which Approach Actually Fits You?
- Do you read 10-Ks and quarterly transcripts for the companies you own, or mostly rely on a screener and headlines? Deep research supports more concentration; lighter research favors more names or an index fund.
- Has your stock-picking historically beaten a simple index over multi-year periods, net of the extra risk taken? If you don't honestly know the answer, that's itself informative — it usually means diversification is the safer default until you do.
- Could you tolerate one position — the one you're most excited about — going to zero? If a single-name blowup would be financially painful beyond "an uncomfortable year," your portfolio is more concentrated than your risk tolerance supports, regardless of which camp you intellectually prefer.
- Do you actually track your position sizes as a percentage of the whole portfolio, or only think about what you originally paid? If it's the latter, drift is very likely already happening below your radar.
Build Whichever Portfolio You Choose — Just Monitor It Deliberately
Whether you land on 8 stocks or 80, the discipline that actually protects you is the same: know what percentage of your portfolio each position represents, and get notified when that changes materially instead of finding out by accident. A price or percentage-move alert on your largest holdings does that watching for you.
Screen for new ideas — however many you ultimately decide to hold — with Stock Alarm Pro's screener, or explore live market data with the no-signup market explorer before sizing your next position.
This article is for educational purposes only and does not constitute investment advice. Position sizing frameworks including the Kelly Criterion involve assumptions and estimates that carry real uncertainty, and both concentrated and diversified portfolios carry risk of loss. Past performance, including any research findings referenced here, is not indicative of future results. Always consider your own financial situation and consult a licensed financial advisor before making investment decisions.