Roughly 50% of all venture backed startups in the US have an SVB account. So yes, it's much larger than just YC.
I think a lot of startups are in this situation. They have SVB accounts, but likely don’t have all of their money in there.
One thing to remember, SVB is very founder friendly. Often giving companies favorable lines of credit, giving founders favorable mortgages, etc. They helped us tremendously with our PPP loan, and are also deeply integrated with products like Stripe Atlas.