10.25 inches of space per slot, 4 slots.
You can do the math how many cards will fit.

PSA standard slab thickness is .25 of an inch, that comes to 40 per slot.BGS has standard slabs at .31 inches thick, that is about 30 per slot. SGC standard slab is .28 so you'll get 35 in per row! Of course there are thicker slabs for thicker cards and they range from .35-.65 inches per slab for all graders, so adjust your expectations accordingly to the cards you have.