How do I know my total available space monthly?

You can view this information in the Account Usage section. Here's the link: https://www.jotform.com/myaccount/usage

As you can see, the current upload space and the limit is showing in the Account Usage page.

Please do check this related guide for more information: https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits

Hope that helps. If you have any questions, let us know. Thank you.