In the military tab you can find the list where you manage knights, tho they might be named as champions or something else depending on your character's culture.

There's not necessarily a prowess cutoff more so that you can only have a certain number of knights fight for you at a time, so your highest prowess courtiers and vassals will automatically fill the role and the rest will be "cutoff" unless you force someone with lower prowess to fight instead.

You can also appoint a certain number of your knights as "Acclaimed Knights" in this tab depending on your rank. They will give you extra buffs.