The AppleShare concurrent user limit is hard-coded, based on performance
assumptions made about the number of users a given type of server could
support. Depending on what type of Macintosh you have and its quantity of
RAM, a certain amount of memory is allocated to support that number of
users.
A 68000- or or 68020-based server with 1MB of RAM supports up to 25 users --
but a 68020 or "higher" CPU with 2MB or more of RAM lets AppleShare support
up to 50 users.