You know, this is one of those "it depends" situations.
And what it depends on is WHAT seedlings are packed full in that jug. For plants that will ultimately remain small (dianthus, allysum) you can do what you have said, but you'll get more coverage if you make the HOS's a bit smaller and leave some space between.

For larger plants, I do try my best to get no more than 3-4 seedlings in a chunk. Because when they grow, only one or two are going to make it that close togther (usually). Yesterday I planted out African daisy. They will get 1-2' tall. I tried to get only 3-4 seedlings in a chunk and planted them about 6-8 inches apart. When thye grow they will be tight - head to head - and that's they way I like them for fullness and weed prevention.

Six or seven seedlings a hunk is about what I do.

Wow, glad i asked, i was thinking 20-30 seedlings per hunk! Omg am i going to have ALOT of extras! Lol

But remember you don't HAVE to stress yourself to get only a few seedlings in a chunk. I think every WSer has planted an entire "brownie" from the bottom of a milk jug in desperation at one point or another! You just get more for your WSing buck by going smaller if you can....

It depends. If I just want a nice patch, I divide the jug into 4 brownies. Sometimes i halve those quarters depending on germination.