File sharing limitations

I have an office of 10 computers, 5 of which use advance
computer drafting software. I have a file sharing
computer with Windows 7 professional that is sharing
files to everyone on the network. The network goes down
almost daily. Nobody can access files during this time.
Rebooting the file sharing server fixes the problem.
Correct me if I'm wrong...The max number of users that
can access the shared drives on the computer with windows
7 pro is 20. We only have 10 users. The 5 computers with
advance computer drafting software open 10 to 15 files at
one time for one drawing. Does this have any effect on
the number of users that can access files on the server?
Any help on this issue would be greatly appreciated.