Note that I run the accountant's version for my own needs, but I'm trying (unsuccessfully) to help a client with minimal bookkeeping needs, but 3 different companies, to get things set up.

Your client would need to purchase one Sage 50 program and can run multiple companies. If they purchase Pro they would be in single user mode all the time. Premium can be expanded to four users and Quantum would go from 5 users and up. A boxed program could be purchased from Staples or similar and utilized for as long as they want until more features would be beneficial. If they require Payroll, a service plan with payroll would have to be purchased from Sage. This would then switch from a perpetual license to a subscription which would be renewed annually to maintain the payroll information current. It would also provide all of the updates to the basic Sage 50 program  as well.

Hope this helps

Alwyn is correct, there's currently no limit to the amount of company databases you can create and or open with Sage 50 CA in any edition (Pro, Premium or Quantum). You can open the program multiple times for different database / companies at once:

You might want to make sure that your 1st level phone support / sales people know this, as someone there said "You need to buy a separate desktop licence for every company file you want to use."