Well, when you have five or six schools, you have to pick what order you want to do them in. So people look to optimize it. Will your essays get better the more schools you do? Probably. But if you spent the same time applying to two or three schools that you'd use to apply to five or six, your essays are probably going to be as good as you can make them anyway.
So anyway, I wouldn't worry about not having more schools because of that. The real worry is that admissions is somewhat a luck based thing, and applying to five or six schools drastically reduces the chance you'll simply get unlucky. If there really just aren't other places that you CAN apply to, then there's no sense in worrying about it.
To answer your question directly: I'm not sure there's really any special strategy for doing two or three schools versus 4-6 schools. Spend lots of time and love on those applications and make them as perfect as possible, I suppose. But ideally you'd do that no matter how many schools you're applying to.