what is the upper limit for UID is AIX 5.3 ?
Normaly you are limited to the Maximum number of fixed Licenses for users to log in to the system but I do not know if there is a number of users UID
to see what you have set on your system look at
smitty chlicense
( this can be made more but Y would you need to have that many user )
this can be a security risk having to many users