There was a thread about this at some point. The answer was "a bunch", though the numbers were complicated. Some had merged, others experienced "exits" which were really just talent acquisitions and shut down all operations on their product. Very few really "failed".

The 9/10 startups thing was always a myth, though, especially if you're talking about startups which get to the stage of getting any funding at all. It's more like 1/2.