Ten. I don't get why this is a constraint.

If they're rubberhosing me, I'll just give them the master pass. They won't have to take my finger.

If they get my fingerprint some other way, I can just switch to my next finger. And when I'm done, I can use PINs.

I don't think it's mutability that matters. It's just that I leave fingerprints everywhere.

I'm pretty sure you can register the same finger over and over again for new "keys". Assuming you go through the tedious process of disabling, restarting, etc whatever each one of the underlying OS demands. It's an additional layer that Bitwarden (and other apps) do not get direct access to.

So no excuse if you value convenience, PINs are not good. Short easy to remember and enter PINs are also the reason Apple is under fire due to how easily you can avoid biometrics and just use the 6 digit PIN.