No more than 2 slots available for services

Unfortunately, Microsoft Bookings doesn’t have a built-in way to limit the number of overlapping appointments across your whole team, it will always offer slots as long as each staff member is free. If you want to make sure no more than two people are booked at the same time, the best workaround is to use a shared resource calendar that represents those limited slots and have customers book against that instead, then assign staff internally. Another option, if you’re comfortable with a bit more setup, is to use Power Automate or Graph API to automatically block additional bookings once the limit is reached. It’s not as simple as ticking a box in Bookings, but those approaches can help you manage concurrency.

------------------------------------
Don't forget to mark as solution if my answer suits you