does the free account give you 10 purchases per month, or total?

I presume you are referring to the 10 payment submission limit for "Free" plans? 

The submissions limits are all counted per month, they reset every 1st day of the month. For "Free" users, you can receive 10 payment submissions on a monthly basis (forms having payment integrations on them and received completed payments/transactions).

Please do note that when you reached ANY of the submission limits, then all of your forms will temporarily be disabled, you can continue using them by upgrading to a higher subscription plan OR wait for the submission counters to reset.

Do let us know if you need more information on this.