How Many Stocks Do You Need for Diversification? Look at What Could Cause Losses
The number of holdings alone cannot tell you how diversified a portfolio is. Learn to map overlap between stocks and ETFs, your largest positions, and shared revenue drivers.
Diversification depends more on position weights and shared risks than on the number of stocks. Even if you own many stocks with different names, their sources of loss are not diversified if they would all suffer when the same customer cuts investment. Rather than setting a sufficient number of stocks for everyone, it is more useful to calculate the loss a single event could cause in your account.
Turn account statements into a list of underlying economic exposures
First, bring together the market values of all your accounts using the same date and currency. Rather than listing each ETF only by its product name, use the holdings disclosed by its manager to check for overlap with stocks you own directly. Adding the number of stocks in a small account to those in a large account as if they carried equal weight conceals concentration. Assets you do not regularly trade, such as pensions or company stock compensation, can also create exposure to the same company, so identify them separately.
Record weights and loss contributions alongside the number of stocks
Record each stock's market value as a share of your total financial investment assets, along with the combined weight of your largest holdings. Multiplying a stock's assumed percentage decline by its portfolio weight gives its contribution to account losses, assuming other assets are unchanged. Add shared factors such as major customers, raw materials, and interest rate sensitivity to see whether differently named stocks are effectively tied to the same scenario.
Ask what an additional holding actually changes
Before adding a stock, explain whether it brings a source of returns absent from your existing holdings. Simply buying another competitor in a familiar industry may spread company-specific risks while leaving the risk of an industry downturn intact. On the other hand, owning too many stocks creates the cost of being unable to keep up with their disclosures. Rather than expecting diversification to eliminate losses, check whether your portfolio is structured so that one company's failure would not disrupt your entire financial plan.
On your review sheet, write one sentence beside each holding describing the situation that could hurt it most. Repeated sentences across several rows signal overlapping risks, regardless of the number of stocks. Even when adding an index product, check whether it increases your existing exposure to large-cap holdings, and do not assume that many small positions sufficiently offset the risk of one large holding. This kind of list explains actual diversification better than a ranking of returns.
Worked example
Hypothetical example: In a KRW 10 million account, you own KRW 2 million of stock A directly and a KRW 4 million ETF with a 15% allocation to A. Your effective exposure to A is KRW 2 million + KRW 600,000 = KRW 2.6 million, or 26%. If only A falls 40% and other assets are unchanged, the account loses KRW 1.04 million, or 10.4%. A long list of products on the statement does not eliminate this overlap.
What to check
- Have you combined holdings inside ETFs with stocks held directly?
- Have you calculated the weights of your largest holdings across all accounts?
- Would the new holding lose money for different reasons than existing holdings?
- Can you actually keep up with disclosures for the stocks you own?
Sources and primary materials
This is educational material about research methods. For an actual company decision, verify the filings and primary sources for the relevant period.