The 800-series router is already End-Of-Sale. The 850/870 are the new generations of ISR's and these should be able to handle IPSec/VPN for 32 or more users without any hassles.

However, if you are still interested in slightly higher, please refer to the following links:

Integrated Service Routers Quick Look

http://www.cisco.com/web/partners/downloads/765/tools/quickreference/isr.pdf

Portable Product Sheets - Routing Performance

http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf

Your performance is likely to be limited by the bandwidth of your ADSL. As long as the router effectively supports that much bandwidth, shouldn't matter whether there are 1 or 10,0000 users. (Of course, the more users, less bandwidth each gets if fairly shared. So, for more bandwidth, for more users, bigger router. [If you wonder how much bandwidth should be available per user, "that depends".])