Diversification in numbers
Owning more stocks reduces the risk that one company's problems hurt you. The benefit shrinks quickly:
| Stocks held | Rough effect |
|---|---|
| 1 | All company-specific risk |
| 5 | A single failure can cost 20% |
| 10 to 15 | Much of the single-stock risk removed |
| 20 to 30 | Most removed; you mostly carry market risk |
| 100+ | Very close to an index |
The trade-off
Fewer stocks means each good idea matters more, but so does each mistake. More stocks means smoother returns, closer to the market, and more work to follow.
A practical answer
- Beginners: an index fund, plus a few stocks to learn.
- Active researchers: 15 to 30 stocks you can actually follow each quarter.
- Check real concentration with the Herfindahl index (concentration): 20 stocks where three are 60% of the money behave like far fewer.
How much goes in each name matters as much as the count. See Position sizing.
See also
- Position sizing How much to put in each investment: equal weighting, conviction weighting and risk-based sizing, with simple rules that keep one mistake from sinking a portfolio.
- Herfindahl index (concentration) How the Herfindahl-Hirschman index measures portfolio concentration, how to calculate it from weights, and the effective number of holdings it implies.
- Index funds What index funds are, why low-cost index investing beats most professional fund managers over time, and how to use index funds as a portfolio core.
- Portfolio vs watchlist Why keeping a portfolio of stocks you own separate from a watchlist of stocks you are studying leads to better decisions and fewer impulse buys.
Pages that link here: A long-term investing philosophy, Building your investing knowledge, Investing FAQ, Market leadership cycles, What is a stock?
Last updated September 30, 2026. Education only, not investment advice.