Probably the most important concept, pertaining to billing, is understanding the difference between a user and an active user. Adding someone to an app’s whitelist (a way to share an app) does NOT mean you need to have an applicable license for them [yet]; only once a user launches/syncs an app, are they counted as an active user–this is when they need license coverage. So, keep that in mind, if you have a whitelist with 20 folks, but with your best estimates, only around 15 will be using the app, you only need 15 licenses (until the 16th user comes around).
Next question–if you subscribe to 10 licenses, and you end up with only 9 users, you will still be billed for the number of licenses you have selected to subscribe to, in this case 10 [on your monthly or annual renewal date]. If you end up with the inverse situation, and have 11 users on a 10 license subscription, the system will automatically alert you to upgrade the number of licenses subscribed to. << This is why I wouldn’t necessarily suggest going and upping your license count EVERY time you add a new user to your whitelist, as the system will kindly ping you when you do need to upgrade (and it’s prorated).
Doest this help clear up the confusion?