i have done a lot of projects beyond 20k content pages and kirby did not even break a sweat. it really depends on how much data you load in a single request. but even then anything up to 500 content files in a single request should be no issue at all.
if you want to build very complex relations via a id based taxonomy and lots of relations then you might run into issues with kirbys panel and un-cached blueprint resolution. think of having multiple tag fields and multiple related pages per page… not a typical website at all. even with a database ORM you need to do lazy loading then.
but all of these are solvable issues once you know what is going on and the forum staff and other members will always be there to help you.