How many user accounts can I have in one Domain?

A. The real problem is that each user account and machine account takes up space in the SAM file, and the SAM file has to be memory resident. A user account takes up 1024 bytes of memory (a machine account half as much), so for each person (assuming they each had one machine) would be 1.5 KB. This would mean for a 10,000 user domain each PDC/BDC would need 15MB of memory just to store the SAM! Imagine a network with 100,000 people. This is one of the reasons you have multiple domains and then setup trust relationships.