I only want to have a form for a month. Can I make a single payment?

I have already made my form and sent it out.   I didn't realise there was a limit to the number of forms that could be submitted before I was charged.  This form will only be used for a maximum of a month and I am happy to page so i can receive all the responses but I need to be certain that I can make a one off payment and then cancel.

You can indeed have your subscription active for a single month.  Upgrade to the subscription you need and you can cancel right away to ensure it is not automatically renewed:

https://www.jotform.com/help/341-How-to-Downgrade-Subscription-Back-to-Free-Account

You will retain you paid subscription benefits until the end of the time that was paid for.  At that point you will automatically be downgraded back to a Free account.