You can do this by making 3 different variables:
- Total seats
- Registered seats
- Open seats
You can store these in an entity and using a microflow you can apply logic to it, for example:
- Total seats is filled while making the registration object
- Registered seats is incremented each time someone registers
- Open seats is calculated by subtracting Registered Seats from Total seats and using the outcome of that to fill / update Open Seats