What is the maximum number of users that can be created in LDAP and WTUSERS?

I don't think this is a limitation of the software as much as it is the underlying hardware.  Open DJ (Windchill DS) has been tested with 3 billion users.  There are limits to search result sizes, but these can be adjusted if necessary.  (https://ludopoitou.com/2012/04/10/tips-resource-limits-in-opendj/)

Same goes for the database table.  SQL Server supports unlimited rows in a table, you just need the physical storage space to support it.