That's a bit of a complex question that depends on several factors: what Mac you have, how powerful is the Mac processor, how much RAM memory you have on the Mac, and finally, how resource-intensive are your applications. In general, a single Mac can support quite a few simultaneous users. However, if you notice that the applications are responding significantly slower than normal, you probably have too many simultaneous users.

If you're running any version of OS X 10.6 or below, then we recommend having a minimum of 0.5 GB RAM per user, with an optimum of 1.0 GB.  

However, if you're running OS X 10.7 LION, it requires a larger amount of memory per user session.  We recommend at least 1.5 GB RAM per user, with an optimum of 2.0 GB. 

So if you take that in consideration, the Mac Mini would allow a minimum of 4 simultaneous users or a maximum of 15 (if you use the 8 GB RAM configuration) if you're running Snow Leopard.  It could allow a minimum of 2 simultaneous users or a maximum of 5 if you're running Lion.

The Mac Pro goes up to 32 GB in memory and its CPU performance is much better as well. The new Mac Pros actually support up to 64 GB of RAM, (http://www.coderebel.com/2010/08/20/mac_pro_ram) so it could support a strong 120 concurrent user sessions without any issues on Snow Leopard, or 42 user sessions on Lion.