Implementing Feature max limit (Logical)
Let's analyze how much effort COUNT(*) will take...

For those numbers, either way will probably work.  Counting 500 rows is likely to take only a fraction of a second.  If you don't need to do that a million times a day, you probably won't run out of performance.

Either approach requires performing some form of test every time the user takes a ticket, so this does not seem to be a limiting factor.