We generally run about 6 large buildings, maybe one or two smaller, and some scatter on a 4x4 table. The only time I've ever had terrain be an issue was on this board:
The 7 biker list was run on that board, but we removed about 4 of the large buildings and spread the rest out nicely. That board just has
too much terrain on it. As it was in that shot, a single biker got stuck (you can see him, about halfway up the table, trying to squeeze between two buildings). After removing 4 buildings, there was still plenty of terrain, but bikers could actually move about.