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 DaySubscriptions Added (cards)Subscriptions Deactivated (cards)Cards that got chargedCards with no transactionCards Considered for billingTotal cards that will be billed at end of the monthNotes
1st Jan - Jan 10th10010000Cause 10 cards are free
11th Jan - Jan 15th502322Only 2 cards have charges above the 10 free limit
16th Jan - Jan 31st55 (from the first batch)0502Although 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.