The number of API-calls-per-transaction depends mostly on how many items your average customer places in his/her shopping cart when purchasing your products. For example, if your average customer puts 3 or 4 items in their cart, your total API count would breakdown as follows:
1.) Each item placed in the shopping cart will trigger a Lookup API call. This call tells TaxCloud to lookup the tax amount of each item, based on where it's being shipped to/from, how much it costs and any special taxability rules that may apply.
2.) If the customer subsequently purchases the items, TaxCloud will get a single "Capture" API call to tell us the official moment of taxation has occurred.
3.) You may also decide to use our Address Verification API (which we highly recommend) to ensure your customer has entered a valid destination address. This enables us to provide the most accurate sales tax information. Note: Without a valid shipping address, you may inadvertently under-collect sales tax and be liable to pay the difference.
So, most business have between 4 and 6 API calls per transaction.