Avoid letting one outcome decide everything
If a portfolio holds one company, that company's setbacks become the portfolio's setbacks. Holding several companies reduces that single-company dependence. Spreading across industries, regions and asset types can reduce other concentrations too.
Diversification does not require every investment to succeed. Its purpose is to prevent one disappointment from doing all the damage. The gains from some holdings can offset losses from others, especially when their results are not tightly connected.
Ten names can still be one bet
A portfolio of ten banks is less exposed to one bank, but it remains heavily exposed to conditions that affect banking. Twenty technology companies may share sensitivity to interest rates or investor sentiment. Counting holdings is not enough; what drives them matters.
Correlation describes how returns tend to move together. Assets with imperfect correlation can improve diversification. Those relationships are not fixed, however. Investments that usually move differently may fall together during a broad crisis.
Protection has limits and trade-offs
Diversification cannot remove market-wide risk or guarantee against loss. It may also reduce the benefit of being heavily concentrated in the eventual best performer. That is the trade-off: less dependence on any single outcome, in exchange for giving up some extreme possibilities on both sides.
Costs, overlap and complexity matter. Several funds can own many of the same companies. A long list of holdings is not automatically a well-spread portfolio if the underlying exposures repeat.