What’s the maximum number of stocks one should have in a portfolio?

I get that diversification is good. But people seem to say that you should have like twelve stocks in your portfolio, and certainly not more than 30. Is that really sufficient diversification? What's the point of mutual funds and ETFs then? Some ETFs have thousands of different stocks. What if I want to replicate a mutual fund and skip a few of its holdings? Is it reasonable to have like 200 different stocks in my portfolio, each handpicked, bought and then held forever?