multiple copies of same app

This is less applicable in today's QuickBase pricing plans than it used to be.  In the current environment, your extra users will be added to your Plan on the next cycle automatically unless you manage them properly.  This was not necessarily so historically.

Since you are building, managing and maintaining the Application/Account there is no conflict; since you own it and can control access to it.  As long as the fees your clients pay you includes QuickBase and includes your management/maintenance services; then you should be free to charge as you see fit for their access to your system.