If you've sold all available tickets for a session and want to stop further bookings, you don't need to reduce the Maximum Event Attendance by one.
For example:
Maximum Event Attendance: 5
Tickets created: 5
Bookings received: 4
If you only want 4 people to attend and don't want to sell the final place, mark the session as Full instead of trying to reduce the Maximum Event Attendance from 5 to 4.
To do this:
Go to Calendar.
Find the session.
Click the three dots (⋯).
Select Update Availability.
Under Availability Status, select Full.
Save your changes.