The user or (connection) limit for Windows XP Pro is 10. Same goes for 2000 Pro and NT4.0. These are for concurrent connections. XPPro does not require user licenses.
You can add as many "local" users as you like to the MAIN computer and if you add identical users to the other machines they can seamlessly access the network shares added to the MAIN PC.
If you are talking a Windows Server SBS or Standard Server the IT rule of thumb is whatever you have the least amount of is the number of CALs required. IE. 10 users with 5 PCs then get 5 DEVICE CALS. Conversely, if you have 5 users(logins) and 10 PCs then get 5 USER CALS.
Hope this helps.
