What would happen if I reach the 500 Total Submission Storage limit?

Hi, once I get to 500 submissions of the form does that mean it is no longer free, or can I delete those submissions?

You won't lose your free account regardless if you hit your limits or not. When you hit the 500 Total Submission Storage limit, your account will remain in the free plan and will never be locked. What happens is that all your your older submissions will be inaccessible from your Submissions Page to give way for new submissions. Bottom line, on the free plan, you can only have a total of 500 Submissions (max), at any given time.

Yes, you may free up some space by deleting submissions. However, be mindful that this process can not be undone.

Here's a snippet from the guide I'm linking below:

When deleting submissions individually, you will see this warning message (see screenshot below). Please note that deleting submission data is irreversible and will permanently remove it in our servers, including the file uploads on them.

Complete guide: How-to-Delete-Form-Submission-Data 

So I strongly urge you to make a backup copy of your Submission Data just in case you'd still need them. There are plenty of ways to do that, feel free pick one that will work for you:

Related guides:

How-to-export-all-of-your-data-at-once

How-to-Download-Form-Submissions-as-Excel-CSV-PDF