different question to thread title - do you mean device group (in theory unlimited, but practically in the order of a thousand or so, it depends on how fast your server is), or machine (around 2000 in standard trim, but can be extended to many thousands, but you'd be really not thinking properly to actually do that).
The less users you have per machine, the faster your network will be, and the more secure the system is. If you do what some people have tried (assign everyone to every machine), you're bound to run into problems in the future, either security wise, network overload, or just plain old management problems.
I know a lot of organisations that think everyone logs onto every machine, but in reality of course, they don't. That's what AutoDomain is designed for.
Also, I have to correct you on your previous statement. You CAN assign individual users to machines. Go to machine properties, go to user tab, click add, then choose group name on left, this will reveal individual users in right pane, select user ... and you're done. The individual user is assigned.
AutoDomain was designed to solve this headache. An admin shouldn't haven to manually assign users to machines. That just isn't scalable. However, if you fill your database with users, you can use our API to automatically assign the right users to the right machines. This happens during the installation if you use the AutoDomain script.