I've used five RK slots out of 25 available on my daily driver keys. Three of those are for SSO providers that provide access to the majority of the other apps I use. I opt in to "sign in with Google" etc wherever I can.

I don't see this impacting me personally anytime soon, but that might change when more sites start insisting on rolling their own RKs.

Hopefully future Yubikey models will ship with more flash, enabling many more RKs if you so desire.