Hello and welcome to the forum.
Interesting question / issue.
I've never heard of a local user account max number limit but of course I've also never heard of anyone creating 100 + local user accounts on a workstation either.
Is this 7 Home Premium, Pro, Ultimate or another flavor?
If it is pro, ultimate or enterprise have you tried using the local user manager snap it rather than the one offered in Control Panel.
just click start and type
lusrmgr.msc
and hit enter
How much space do you have on your C:\drive even a minimal user profile can still take up a bit of room and a C:\ drive that is near full may account for some weird behavior.