I count 9 successful projects so far, as in things that made money and things that users used. They may not still use it but people paid and were satisfied.
I guess the way I see it is you fail some things and succeed others. It's similar to being a professional athlete; you don't plan on winning every game, just need to win enough of them.
Founders of failed startups do other things. They get jobs, get fired, hold CxO or VP positions somewhere, quit and start new things. Some work corporate, some do other startups, many join unicorns who usually have good cultural fit and pay.
Currently I'm at a YC-funded unicorn, working on the kind of projects that make 30% month on month growth. Experience dealing with product, architecture, tech debt is very handy.