I believe you are dealing with a software limitation. Check your documentation or search the knowledgebase to be sure. With that many simultaneous users, you should be considering FMServer anyway.
Note: It's a very bad idea to put your database files in a shared folder! You can end up with corrupted files! The approved method for sharing a file using FMP is:
- Launch the filemaker application on your designated host machine. (Files should be stored on this machine's internal drive and not in a shared directory.)
- Launch filemaker app on the client machine
- Select Open remote/click the remote button to bring up the hosts dialog box
- Select your file to open it.