Based on a bit of research, watching the Unite ’07 Performance Optimization talk and talking on the unity forums:
Each material on each mesh causes one draw-call per frame, per light that affects it and per camera that sees it.
If I have 1 mesh with 20 seperate textures, only ambient light, one camera, this will also result in 20 draw-calls. If I would combine the 20 textures into one atlas the scene would now only need a single draw-call.
If I have 20 meshes with one texture each, only ambient light, one camera, this will result in 20 draw-calls. If I would combine all the textures into one big atlas, the scene would still require 20 draw-calls, even though all the objects would now use the same texture.
The only way to optimize here would be to combine meshes. But you can only do that if your meshes aren’t to move around independently from each other.