I want to limit sales

I want to limit sales of tickets on tours like as maximum 20 tickets for a tour. Ticket sales should be blocked automatically when 20 tours have been booked.
I think so there are shortcomings in the settings of “tour booking”. Current settings are:

Regular price (TRY)

Sale price (TRY)

Child price (TRY)
0

Tour Code

Max number ticket (adult) per booking
Set empty for no limit

Min number ticket (adult) per booking
1
min required = 1

Max number children per booking
Set empty for no limit

Min number children per booking

Thank you.