I'm not an expert on fronts, however since I've recently aquired some I've been looking into them alot. I would think no more than 3 fronts in a 120g comfortably.
I have a 120g and I'm thinking that I'm going to get another for my fronts. I really like the fact that the 120 is 24" front to back. It gives the tank a nice option for decorating in 3 deminsions.
My fronts(2) are currently in a 55g. They are doing fine. But I'll feel much better when they go into a larger tank.
I'm sure some others on here can give you more advice based on experience though.