Circus KroneIt says here that it seats 3000 but I think it feels smaller. Seating is tiered (like in a tiny opera house) and you get a good view from all sides (it is circular) along with seating on the flat.
The acoustics aren't bad.
You'll get tickets from all main ticket suppliers,
München Ticket to name just one.
Personally find it a slightly strange line-up but that's just me being picky.