ummmm what do you mean? Are you talking about how many local users on a member server or are you talking about running AD on Win2003 std vs. Win2003 Ent?
From a high level (if were talking Active directory), it is my understanding that AD will support over a million objects (users, computers, OU's, GPOs, Etc....). Microsoft states that a single DC can support 10,000 users, as far as logon goes. Usually ADs are running multiple domain controllers, each usually running Win2003 Std. Running enterprise version as a domian controller is based on Hardware and availabilty options, not the amount of user accounts you are managing. You would usually use Enterprise if you wanted to do an HA cluster on you DC or if your memory or storage requirements are larger than Std. can handle. I have not seen DC's running Enterprise version, but would not be suprised to see it out there...