What is the lowest number of users I can have for my app (in respect to billing purposes) plan?

The Quick Base plans are more based on features and not the # of users.

The plans are listed here, but I have found that for all of my clients, they need the middle level plan "Premier" to get the feature set we need.  The entry level plan is missing some important features.

http://www.quickbase.com/plans-and-pricing

Thank you.  

One other issue to consider is that QB allocates database storage on a per-user basis. You can save money reducing the users you will need, but you might need to buy more app space a la carte.