You can arrange memory palaces however you want. There are some posts about nesting locations here:
I’m not sure exactly what you mean by walls in alphabetical order, but I often arrange locations in rooms like this:
The square at the bottom is the door. Location #9 is the ceiling and #10 is the floor.
If you pick 5 objects in each location for nested locations, you could get 50 locations out of a simple room.
There’s a good post here on choosing the size of locations: The Efficiency of Different Card Memorization Systems - #21 by Boris