Depends what the users are doing.
I had done some earlier torrent testing with a Fonera 2201+
8MB flash, 16MB ram, 200mhz AR2315 CPU (overclocked a little

)
Anyway... with just me using the router, I had 4 torrents running
I downloaded the torrents for 4 dvd-sized linux distros and set them
to a max of 64kbps upload, but no limit on connections.
Across the 4 torrents, I hit 200 connections, and the router had
slowed to a CRAWL... I still had plenty of upload/download
bandwidth, but all those connections just killed the router.
Obviously the Fonera 2201+ is no powerhouse, but it was meant
purely as a test to show the # of connections can tax a router.
I'll need to re-try this test on a WRT54G-TM, and see what kind of
results I get as well. I suspect a bit more connections because
of the 8MB flash and 32MB ram, but its still only on a 200MHZ cpu
as well, so I'm guessing I'll bring it to a crawl at around 300+
connections, maybe upto 400 connections.
That being said, for 100 users, typically just 1 user can use 10-20
connections easily, and if all 100 were using it at the same time,
you'd be looking at anywhere from 1000-2000 active connections.
That poor WRT54G-TM would probably cry and throw a hissy-fit.