Is it possible to cater for the following scenario:

Software with a costing model of a set number of uses on a periodic billing cycle.

Can subscriptions cater for a plan with a limited number of sessions?  I assume that a custom field will have to be created on the Customer record to keep track of the sessions (custom field to be updated by API calls from the software as the customer logs on to keep track of number of sessions used).  

Or is there a better way?

thanks

chris