I do not know if there is a difference between the ZXA1 and the 90B, maybe just a description thing. I rent my gear and had the ZXA1 with ZXA1-sub combo and used 2 speakers with 2 subs (one below each speaker with a mounting pole in the sub to mount the speaker on top).
But you can mix and match the ZXA1 series however you like, for your room 2 speakers and prolly even one sub would be sufficient. You can go with 2 subs from the start if you like to use it outside the room for mobile gigs too.
With max 20 people you will have LOTS of reserve and headroom, but the sound quality of those speakers is awesome.
Of course if you prefer JBL (Eon Series) and HK (One series) have similar offerings at that price range.