Worth noting that cavalry warbands work better when only 6 big: there is less room taken up by large bases blocking combat, and more might spread across the table

But yeah you can have up to 12 in a warband.

And you wouldn't have enough Might or Standfast for 12 mounted warriors.

_________________
"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

This is a good point. I think one of the few (if only) cavalry that can be effective with more than 7-8 in a warband would be Dol Amroth, due to Imrahil's banner effect. For the rest, the heroic bubble isn't really big enough. (RoR might be another exception, because you don't need to be in a bubble to snipe.)

Honorable mention to morgul knights led by ringwraiths, the 12" harbinger of evil makes them tricky to counter charge so taking 8-12 is do-able.