If done right, I like the look of a crowded shelf. The trick is to make sure every figure can be seen. Risers are a must. Knowing the point where the shelf gets too crowded is a "you know it when you see it" moment and, of course, depends on the shelf dimensions. I can fit 20-30 figures (mostly Deluxes and Voyagers, but some bigger and smaller figs for balance) on a 30-inch wide shelf. I've got nearly 50 G1 Diaclone carbots and Minibots on a single shelf, using risers, and everybody can be seen.