The only router performance figures that Cisco typically gives are packets-per-second (pps). A more useful metric is maximum bandwidth (i.e., how much traffic can the router route), but these numbers are only available from third-parties or end-users who have run bandwidth tests on their own.
It sounds like your question is essentially a bandwidth question -- how many users can the router handle. In order to calculate such a number you need to know the average bandwidth utilization per user and the total amount of bandwidth that the router can handle. The former is for you to calculate since you know your user base, but it's still a very difficult number to come up with. The latter is somewhat easier: a 2621, rated at 25kpps, can do about 40Mbps with full-sized packets per some testing that I did a while back. The 2611XM is rated at 20Kpps, so I'd guess it can do somewhere between 30-40Mbps. It's impossible to know for sure without testing, though.
