Keep them to three songs or have time slots. Monitor the buggers because you can be guaranteed that if there's no adult supervision, 50% of them will play a fourth song.
The one open mic I go to that I'm not house bassist for[1] squeezes as many acts in as possible, and does this by keeping it to two songs apiece in ten minute slots. Others go for 15 minute slots. Changeovers need to be pretty rapid, which is bound to include moving furniture like stools and chairs onto and off the stage[2].
How big is this place that you're talking of subs? I take my 10" Deltalite cab and Tecamp Puma along to the ones I'm house bassist for, and there's a vocal PA. Acoustic guitarists go through the house PA (straight to desk, no mucking around with DI boxes) and electric guitarists will invariably bring their own amps for "their own sound".
[1] Anyone that wants a proper bassist asks me, otherwise Tom is liable to perpetrate something bass-related on them. Just imagine someone who has generally got lost by the second verse of "Stand by me" and who plays "Folsom Prison Blues" with a walking bass line (which also gets lost around the second verse).
[2] The term "stage" applies simply to the zone within which an act will perform, with no implications as to height, isolation, etc