DriveWatcher pricing is calculated based in the number of subscriptions purchased for the team, not by its number of members.
If you are using the Free Plan of DriveWatcher, you will not be billed. However, you will not be enjoying all the features DriveWatcher provides.
Once you add billing, you will be either in Team Plan or Domain Plan. In these plans, the price is 0.5€ per subscription per month. Only active subscriptions are counted when calculating the price each month.
In case you enable the «Auto-increment total subscriptions» setting, you will be able to create subscriptions without a limit, and you will be billed according to the maximum number of active subscriptions during the month.