How can I setup a field that will automatically fill out a slot once a certain limit is reached

I'm trying to find a solution for our child safety seat check registration.
We wanted to set payment to each registration by time slot. I trust that
your form won't have any problem to setting a limit to a time slot, let's
say a 9 am slot can have up to 3 person signing up, and then that slot will
automatically shut off.

What you're describing can be achieved using widgets. There are plenty to choose from so I'll just list them all below. These widgets track the number of submissions and automatically reduces the number of slots/options/items for each. As soon as it reaches 0, the slots/options/items will be disabled and can't be selected by anyone anymore.

🔘 Appointment Slots Widget

🔘 Inventory Widget

🔘 Ticket Purchases Widget

🔘 Gift Registry Widget

🔘 Quantity Gift Registry Widget

Related guide: How-to-Add-a-Widget-to-your-Form