12 floors is the maximum. If you need more it cannot be achieved with autogen, you have to design a 3D scenery object.
However about autogen, my undersdanding is that FSX/P3D is interpreting the building density in the vicinity and also the current building footprint surface to select final building height.
If you request maximum heights for your autogen buildings area, but it's a region with small towns, you have no chance to reach 12 floors.