Bro, actors can play multiple roles. I can't think of an actor that wouldn't love that opportunity.
And you're only shooting short films at the moment, so what in the hell could call for MORE than five?
What do you mean you can't think of anything with just five people?
Write a romantic scene, how many does that need? Two? Three if you count the waiter, or limo driver?
Write a scene involving friends hanging out, when suddenly a home invasion robbery takes place. Doesn't need more than three people, five would be fine.
Write a shootout scene between two cops and three meth dealers.
Write a scene involving 5 people going camping, when an axe murderer kills them one by one (Have whoever isn't in the shot at the moment be the axe murderer, or be it yourself)
Write a road trip scene. 2 guys travel somewhere, stop at three different places, and meet each of the other wild and wacky characters at each place, and they join in on the adventure.
Write a scene about five people playing russian roulette.
Write a scene about the president preparing to give his state of the union speech, when his wife keeps calling and asking if he has done the mundane chores of the day (Picking up the dry-cleaning, taking the kids to the arcade)
How have you tried very hard and been unable to come up with something with just five characters when I gave you some stuff I thought of off the top of my head in two-three minutes, which, if written well, would look great in your portfolio, and possibly win you some awards (And cash... that's what it's all about
MMmmm... tasty, tasty cash...)
Watch these guys's TV Show.
http://en.wikipedia.org/wiki/The_Whitest_Kids_U'_Know
They did five seasons filled with sketches/short films with only five actors (who were also the writers)
They made an entire movie/miniseries for season 5 with mostly just them playing every role.
Two of them made a movie called Miss March that mostly involves the two of them, without much input from other actors, and if so, not more than two per scene.
This shouldn't be difficult. Nothing is impossible. Some things are just harder than others.