You'll need to look in <limits.h> (or one of the files it includes, e.g., sys/syslimits.h on OS X) for the #define of UID_MAX.
Most recent operating systems (Solaris 2.x, OS X, BSD, Linux, HP-UX 11i, AIX 6) can handle up to two billion (2^31-2)
Do you think we have a problem with Two Billion UID's as a maximum?
(Edited 11 seconds later.)
Minichan's grand plan has always been to be an exclusive club. Once there are no more UIDs, users will have to trade or buy existing Minichan UIDs. We've all been mining UIDs for years. We're just biding our time until the market is right. Phase II of Minchan's plan to take over the world involves the Minichan MLM membership plan. With a few people controlling most of the UIDs, we can create scarcity and demand whenever we want and ride that gravy train for years to come.