From my basic knowledge of these matters I believe the more you pay the provider, the more max users they will allow you to have on the server. Probably all comes down to bandwidth available, and bandwidth costs money.
The number of users the server
can hold hasn't got much to do with how much bandwidth is used, but you should have an estimate of how many clients the server WILL be holding. I would estimate a server holding 600 concurrent connections regularly would use around 85+GB per month. That is a rough estimate though.
If you're linking to another network they should tell you their minimum number of connections, and their "Routing" team should be able to inform you on the more technical details of setting up a server.
Hope this is some sort of help...
Regards,
