If they are standard pallet size like what's seen in the civvy world, 3 at most.
I drove my truck to work last week to get 3,000 24" x 24" cardboard pads that we use to ship wheel and tires (already mounted) from our local warehouse supplier. I couldn't get the pallets side by side, they were too wide. I didn't measure them, but they looked like the standard warehouse sized pallet. I almost got them 3 deep, but was short a few inches. So technically, I only got two pallets in the truck.