I don't know if there's a limit or not, but after 10 or more users,
the system is going to slow down somewhat just trying to sort through
and line up everything with the identity of the person who is logged
on. Things like the registry, menus, desktop, etc. have to be sorted
through and displayed properly for each person who is logged on.
All that takes time and resources. Not to mention with that many
users how easy it is for the system to get trashed.
With all this user information stacked up in the same place, you MUST
run a defragger. If you don't have a good one, the built in windows
one is OK. This should help, but may not be noticeably faster. Also,
how do you manager to get 50 users on one install of xp home?