Brent, it all depends on who the figures are. The majority of figures (height-wise) take up four slots; taller figures like Chewbacca, the other Wookiees, or R1-G4 take up five slots; Astromechs take up three slots; the smallest space taken up is by Yoda, who uses 2 slots.
On average, each case can hold 12 "standard" height figures, although this can vary. When I'm organizing mine, if I have a tall figure like Chewbacca, I will put one of the short figures like R2-D2 below him, so it ends up evening things out. If you had a box of all astromechs, though, you could have 16 figures in one Plano 3700.
There are some figures who will not fit in the 3700 at all, like Ephant Mon, Shaak Ti (Saga Version), Muftak, and the Darktrooper. As mentioned earlier in this thread, there are other styles of Plano cases and other companies' cases (as Jared posted) that can be used for these larger figures. When it comes right down to it, though, there is a lot of trial and error in figuring out who will fit and who won't.
If you have any other questions, please feel free to ask away as I have so many of these Plano things in various shapes and sizes that even if I can't answer off the top of my head, I can always grab one and do some experimenting to help out.