Sage 50c Pro is single user.

Sage 50c Premium can be licensed for up to 5 users.

Sage 50c Quantum can be licensed for up to 40 users.

I have no idea what performance would be like if you actually tried to have 40 simultaneous users. I've seen many clients have good performance with 5 - 10 simultaneous users. Maybe others can chime in with their experiences with large numbers of users.

Good answer Steve!   If you are looking to purchase new software, Steve and I both sell all versions and business care plans of Sage 50.  We can help you decide what is best for your organization! 

We have some 40 user installs that perform fine -- the best are those running via Remote Desktop.