Joe Wiggins explaining how diversification affects risk.
Although diversification is by no means a free lunch, it is an effective means of reducing and controlling risk, if done prudently. It works because by combining securities and assets with different future potential return paths it significantly constrains the range of outcomes of the combined portfolio.
If we move from a single stock holding to a diversified 50 stock portfolio we greatly lower the potential to make 10x our money, but also (nearly) entirely remove the risk of losing everything.
Diversification is a tool whereby we can (very imperfectly) create a portfolio with a range of potential outcomes that we are comfortable with. When individuals complain about over-diversification, what they typically mean is that the range of outcomes has been narrowed so that average outcomes are very likely. There is, however, no right or wrong level, it simply depends on our tolerance for risk. Or, to put it another way, our appetite for extremely good or extremely bad results.