What is the submission storage limit?

Please note that the every submission received to your forms shall be stored in your account. The number of submissions stored in your account for all your forms is the Submission storage. The submission storage is limited to 500 submissions for free accounts. When exceeded, the oldest submission shall be deleted to store the new submission received. 

https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits

Thanks!

Thanks for that info.

Is there anyway that I can delete any of the submissions, please?

Yes, please refer to this guide https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data on how you can delete your submission.

Let us know if you have any questions or if you need further assistance.