Draw calls, merging, and the batching tradeoff

Each draw call carries fixed overhead: state validation, uniform uploads, driver work. On desktop a few thousand is survivable; on a mid-range phone a few hundred is the honest ceiling. Merging static geometry that shares a material collapses many calls into one, and instancing does the same for many copies of one thing.

The tradeoff is culling. The renderer skips objects outside the camera frustum, and it can only do that per drawable: merge a whole environment into one buffer and it becomes all-or-nothing, so you draw every triangle even when the camera is looking at a wall. The workable answer is usually spatial: merge in chunks that are roughly the size of what fits on screen, so culling still has something to work with.