Hello, @Tutoring11663 - great question!

Yes - you can do this for time slots already booked. For example, if you have set your Group Event Types to allow 10 invitees per time slot (and yes, what you put in that section of the event type settings will apply to all time slots automatically) and then one invitee books for Tuesday at 10am, you can update that time slot’s invitee limit. 

1. Navigate to your Scheduled Events tab.
2. Scroll down and select the scheduled event you'd like to adjust.
3. Select Edit Invitee Limit.
4. From the pop-up, adjust your invitee limit.
5. Select Apply to apply the new invitee limit to your scheduled event. 

This will only apply to that time slot so you’ll need to adjust each individually (and each can be different). If you are lowering the invitee limit you will not be able to make the value less than those invitees already scheduled (ex: if 4 invitees have booked you cannot make the limit <4). 

I hope this helps!