In any given billing period (month) only those cards are considered which have at least 1 actual charge (transaction). If cards don't have any transactions they won't be counted. And of the total cards that are counted, the first 10 are free.
See the simulation Period: Jan 1st - Jan 31st.
|End of Day||Subscriptions Added (cards)||Subscriptions Deactivated (cards)||Cards that got charged||Cards with no transaction||Cards Considered for billing||Total cards that will be billed at end of the month||Notes|
|1st Jan - Jan 10th||10||0||10||0||0||0||Cause 10 cards are free|
|11th Jan - Jan 15th||5||0||2||3||2||2||Only 2 cards have charges above the 10 free limit|
|16th Jan - Jan 31st||5||5 (from the first batch)||0||5||0||2||Although previous cards are deactivated there were transactions on that card.|
So as you see although you have 15 active subscriptions, you'll be charged only for 2 subscriptions in the month of Jan.