Monster spawning patterns and mob grinder layout

Most mob grinders I've seen have a broad flat room for the monsters to spawn in. But I saw one recently that was a tall tower instead. That seemed more convenient. If you wanted to expand it you could just add another floor or two. But will it spawn as many monsters?

Does anyone know the rules that monsters spawn by? Do you get a limited number of monsters per x,z map area? Or will monsters spawn above each other in a large tower?

I know they don't spawn within about 24 of the player, but beyond that how are they distributed (along the x, z, and also y axes)?

You are looking at the same constructions. People do the single or few layer layouts for explanation. You then replicate the spawning layers over and over again to increase the area for spawning in a vertical manner so they can all be dropped/directed to your grinder and then retrieval system.

To get into the nitty gritty of spawning rules, you can find them at this post here. There is a lot of good information in there and it explains some of the design considerations in mob factories. Make sure to pay special attention to the changes from the Halloween patch (no more high spawn area, mostly).