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.