I think the limit is as many people as you can cram into a couch. The Controller dongle can handle 4 connections per dongle, and 4 dongles per steam machine. so 16 Steam controllers atleast. not to cast shade but there is a reason why consoles do not exceed 8 controllers, even back with the NES and SNES and their 4x inputs; back then it was a h/w limitation but typically there isnt that many people playing games at once. most games are not accommodated for it either. Putting 15 people on screen can be very disorientating. Sports games like Tape to tape with 5x5 or party games are mostly the audience.

wired limit will be the limit of USB which I think is in the hundreds. I think LTT did a USB limit with chained docks with windows a few years ago. These usb limits are pretty universal.