It's not a dumb question at all. I've asked much the same question in private email to other people. I think it's a combination of how many sheets a given developer will develop, and how long the developer will remain active in the tray.
ABC Pyro, for instance, exhausts in the tray fairly quickly. I was developing only a couple of sheets of 8x10 in a quart of developer - I didn't feel comfortable shuffling more. The amount of developer should handle more, but ABC exhausts in the tray before another set of negatives could be done. Obviously it wouldn't work for doing more than one negative by brush.
I'm told Pyrocat HD lasts in the tray a good deal longer and several negatives can be brush developed sequentially.
I believe HC-110 would work for several negatives, too. I plan to try tonight.
I'm not sure about Rodinal, but the 1:50 dilution seems strong enough to handle several negatives if it doesn't exhaust.
Sorry I don't have a more definitive answer.