So how many does he usually maintain?

Avatars that is.

Probably 2-3 at maximum. Mab has been a bit cagey about how many a Tri-wing can have, and I think it depends on how powerful the Tri-wing is. Skill probably factors into it as well, and Daryil has only been like this for about 25 years, which isn't a lot in the grand scheme of things.
It's also unclear how powerful Daryil would be - each ascension in recent history has been harder and required more power than the previous one. I don't know if that would make Daryil spectacularly powerful because of the colossal energy it took for him to ascend, or the runt of the litter because the process used so much energy up and left only a few scraps to play around with afterwards.

Hence I'm reluctant to pin things down in the absence of further details on DMFA lore.