How Many Stocks Do You Really Need? Why the 40th Stock in Your Portfolio Barely Diversifies

You're sitting with your broker, your portfolio has 28 positions, and you feel a sense of accomplishment. You scroll through the list — Apple, Microsoft, Nvidia, Alphabet, Amazon, a few smaller tech names, an ETF on the S&P 500 "just in case," and two semiconductor companies because AI. Then comes the day when tech drops a few percent — and your "diversified" portfolio drops almost the same. That's when you realize that counting positions and diversifying are two different things.

Diversification Isn't About the Number of Lines in a Table

Many investors believe that the more stocks they have, the safer they are. The reality is different: diversification doesn't decrease linearly with the number of stocks; it drops steeply at first and then quickly exhausts.

Imagine it with a simple example. You have a portfolio worth 500,000 Kč:

1 stock = the entire portfolio stands or falls on one company. If it has an accounting scandal, you lose tens of percent overnight.
10 stocks evenly at 50,000 Kč each = one bankrupt company costs you roughly 10% of the portfolio. Unpleasant, but survivable.
30 stocks = one company is roughly 3% of the portfolio. The collapse of one stock almost gets lost in the noise.

And here's the key part: the difference between 1 and 10 stocks is huge. The difference between 30 and 50 stocks is almost cosmetic.

Data: Where the Diversification Curve Breaks

The risk of an individual stock consists of two parts. Specific risk (the company disappoints with results, the CEO leaves, a lawsuit arrives) — this can be diversified. And systematic risk (the entire market falls, a recession comes, rates change) — you never diversify this, not even with a hundred stocks.

Classic studies on portfolio variance show roughly this: a single stock typically has significantly higher volatility than the entire market. As you add stocks:

The first 10–15 stocks remove most of the diversifiable (specific) risk — you get roughly close to market volatility.
Between 20 and 30 stocks, you're only adding minor improvements — each additional stock reduces the overall risk by only a fraction.
Beyond 30–40 stocks, the gain in diversification is practically zero. Only systematic risk remains, which is common to all stocks.

In other words: the 40th stock in your portfolio doesn't reduce risk by anything measurable. Instead, it adds work with monitoring, transaction costs, and mainly — a false sense of security.

Hidden Concentration: The Trap That Counting Positions Doesn't See

Here's the unpleasant twist. You can have 30 stocks and be worse diversified than someone with 12 stocks. Why? Because diversification doesn't depend on the number, but on correlation — whether your positions react to the same things.

Look at a model "tech" portfolio: Nvidia, AMD, TSMC, ASML, Microsoft, Broadcom. Six different names, six tickers. It looks like diversification. But these stocks historically have high mutual correlation — they often move in the same direction because they react to the same factors: chip demand, rates, AI sentiment. On the day the market reassesses AI enthusiasm, they all fall at once. You own one bet in six wrappers.

And the trap goes even deeper. Let's say you have:

An ETF on the S&P 500 (roughly 30% of the index's weight is now in a handful of the largest tech companies),
plus Apple and Microsoft separately,
and a tech ETF for "extra exposure."

Congratulations — you now own Apple and Microsoft through three different paths. Your actual concentration in a few mega-cap companies is much higher than the number of lines in the overview shows. This is called hidden concentration, and it's one of the most common reasons why a portfolio falls more than the owner expected.

Practical Framework: Diversification Audit in 20 Minutes

Instead of counting stocks, do this exercise. Open your portfolio and go through the checklist:

Calculate real weight, not count. What percentage of the portfolio do your three largest positions make up together? If over 40%, the number of stocks is irrelevant — you have a concentrated portfolio.
Unpack ETFs. For each fund, look at the top 10 holdings. Are there names you already own separately? Add up the real exposure.
Map sectors. Divide positions into sectors (technology, finance, healthcare, energy, consumer goods…). If one sector makes up over 35%, that's where your hidden risk is.
Test correlation with a layman's test. For each position, write one sentence: "What does this company depend on the most?" If the same answer repeats for five stocks (rates / oil price / AI demand), they are not independent bets.
Check geography and currency. 25 American stocks are not currency diversification — it's 25 bets on one market and one dollar.

The goal isn't "to have a lot of stocks." The goal is to have roughly 15–25 positions that are independent — across sectors, regions, and factors. That's the point where you've removed most specific risk, and yet the portfolio is still reasonably manageable.

Control Question at the End of the Audit

Ask yourself: "If one specific sector fell by tens of percent tomorrow, how much would I lose?" If the answer is "almost the entire portfolio," you have a concentration problem — and adding the 41st stock from the same sector won't solve it.

In QMA: instead of manually unpacking who overlaps where, sector and factor filters in the screener show you the distribution of your selection across industries and regions — so you see hidden concentration (like three tech stocks that move the same) before a bad day on the market shows it to you. QMA does this mapping for you.

Takeaways

Diversification is one of the few things where "less is enough" — after about 20–30 independent positions, additional stocks practically don't reduce risk, they just add the illusion of safety.

Stop counting stocks and start counting weights and correlations. Three largest positions under ~40% and no sector over ~35% is a healthier goal than "having 50 stocks."
Unpack your ETFs today — look at the top 10 holdings and find out who you own multiple times without knowing it.
Write down what each position depends on. If the answers repeat, you don't have 30 bets, but three bets in 30 coats.
Check the distribution of your selection across sectors and regions in the screener: open in QMA.

Diversification isn't about having a piece of everything. It's about ensuring that no one thing — one sector, one factor, one company — can sink the whole ship. And for that, you don't need forty stocks. You need forty minutes of honest auditing.

---

This article is for educational purposes only and is not intended as investment advice. Please consult a financial advisor before making investment decisions.

🧩Diversification that Investors Underestimate: Why the Risk Curve Flattens After the 30th Stock

How Many Stocks Do You Really Need? Why the 40th Stock in Your Portfolio Barely Diversifies

Diversification Isn't About the Number of Lines in a Table

Data: Where the Diversification Curve Breaks

Want to know more? Ask the QMA Research Assistant

Related articles

See it live: QMA scores 17,000+ stocks for you

📬 Free weekly QMA Brief

Hidden Concentration: The Trap That Counting Positions Doesn't See

Practical Framework: Diversification Audit in 20 Minutes

Control Question at the End of the Audit

Takeaways