There was once a thread (with math!) that explained that if you have your bakery set up for maximum efficiency, the first customer can get in, sit down, eat, and vacate the table before the 19th customer walks in the door. So even though you get more than 18 customers at a time, you don't actually need more than 18 tables.

(I've never tried it myself, I'm just summarizing from memory and going by your count of 18 tables.)