I have a single stage press and usually do 200 or so at a time. I may break portions of the process down to 50 or 80. For example, starting with 240 pistol cases with primers, I might put powder in 80 and then put the bullets in before turning back to the remaining 160.
It's kind of like when you were made to write "I will not . . ." in school. You may write sentences, words in column or part of both. Or even play with patterns. It can get boring.
In any event, things are usually done in multiples of 50 or 80 or combinations thereof because that's how many cartridge cases my holders will handle.