You also need to worry about the number of concurrent connections, which is roughly based on the number of users but matters more with what they run than how many you have.
One user with bittorrent will need a lot more states than a user who casually surfs the web.
The ALIX only has 256MB RAM (in most configurations, some have 128MB but they are much more rare to see) so you can't allocate a ton of RAM to handling connections. By default it will take 10% of the RAM for that, so 25000 states. Each user connection takes two states (one into the firewall, one out of it), so that's really 12,500 user connections.
If everyone's web browsers use ~100 connections (random wild guess) at a time, then you could have 125 users at a time. If they only take 10 connections at a time, you can get away with 1250 users.