If it's split front end / back end, with front end on each person's desktop and the backend data on a shared drive, the best answer I can give is "several" -- say, 5-10 for decent performance. After that it slows down.
If you're planning on having multiple users, I wouldn't recommend having a single unsplit database.
Tools | Database Utilities | Database Splitter will do the job for you. Follow the wizard, select the network location for the backend and the name you want to use, and let it go to work. Now give every user a copy of the front end.
Advantages:
1. Multi-user
2. You can modify your copy to create new forms / queries / reports while the users access the data. When it's all tested and ready to go, give users the new version.
3. Further down the track, you can upsize the backend to SQL server if you have heaps of users. You still get to use the front end.
Denis