Let's say you start with the Standard Edition which covers up to 100 Monthly Active Users and costs $ 600 for the entire year. With this, what you are actually paying for is 100*12 or 1200 months of usage. If you have 120 volunteers using it in March, Apr and then only 80 volunteers using it for the next couple of months, you would still be Ok.
It's like you are buying 1200 tokens and you can continue to use that till the tokens run out. You can have any number of volunteers set p in V4S Mobile. We only count volunteers who actually use the app in a given month as an Active User for that month. And they can sign in and out of the app any number of times during the month.