Bookings and manually adjusting Max Attendees

Hello.  I have two services I offer on a shared booking.  On the occasional booking, I want to manual adjust the number of available attendees from 6 down to a lower number.  The default is set to 6 on the service page, and on the occasional service I manually adjust it down. 

I noticed on a booking an attendee canceled their appointment, but after that, the number of available seats went from the manually adjust 4 back to 6.  2 extra people booked before I caught it. How can I fix this?  I don't want to keep creating new services for each specific situation, I just want the occasional service to be at a lower max attendee than the others.  Much appreciated.