I don't really know about limbs, but I know from experience with other engines that mobile should only have about 30 drawcalls at the most. PC can handle more, 400 is OK. I don't know about Leadwerks, but depending on how the engine is built, it could be per-material. And it could also be per-lightpass, so if you have 7 dynamic per-pixel lighting one model/material could cause 7 drawcalls. What you want to do is batch models, so they are called in one drawcall. Now that means if you batch every rock in your scene, every rock will be rendered, even if it is not in the camera. So batch models that are close together. So you could batch the entire building, or every few walls.
I don't really understand Chris's obsession with limbs. As far as I know, you can have a model without limbs, and therefore no performance hit. But maybe I am wrong. But yeah, the more limbs, the more performance is going to drop.
Anyway, my rule of thumb is to group models that are close together into one batch. Do not worry about your building being one model, worry about drawcalls. I tend to worry more about drawcalls and opacity then about polycount, but then I come from mobile roots, so you can't blame me