I only want to buy one form how do I do so

Unfortunately, that's not possible. If you want to increase the number of forms limit, you will have to subscribe to any of the paid plans like Bronze. To do that, please head to the pricing page https://www.jotform.com/pricing

If you do not want the form to be counted towards the form limit, simply archive the form that you no longer use. This will disable the form and get lifted from the count.

Here's how to archive a form in My Forms page.