Other answers have responded literally to OP's question regarding specific limits. Given the nature of SF as a long-term reference too, I think it's important to point out a very important caveat on the approach you're contemplating.
You want to be using a directory service to manage user accounts on this scale. It's exactly the problem that directory services [e.g. OpenLDAP, Active Directory, et al.] were designed for.
Using "standard" Unix user tools to manage more than a handful of local user accounts is a well-worn pathway fraught with pain, will not scale very easily and - given your problem description - will in all likelihood lead to a significant re-architecting of your chosen solution when it does not in fact scale horizontally with multiple servers.
 They generally work very similarly, but the exact incantations vary from platform to platform, are different sometimes even in similar heritage Linux distros and of course are regularly changed with OS release versions. Caveat emptor.