Forms do not have a way to track how many times they are submitted, so there isn’t a built in mechanism for this. If it’s not a form where you would expect a lot of submissions at the same time, you could create a data table with 1 field (counter) and put the value of 1 in it. Each time you submit the form, you would run a workflow that would query the table for that number and set that value in the form, and then increment the number and update the table for the next form submission.