A 48-foot trailer can hold 28 standard pallets at a time. Standard pallets are generally 48 inches wide by 40 inches long, so a 48-foot trailer that is 99 inches wide can be loaded 14 pallets deep and two pallets wide.
Trailers are designed to be slightly larger than the pallets they haul to accommodate for space lost when being loaded. This is also why the trucking industry uses a standard 53 foot trailer most often to haul dry goods, allowing more items to be carried on pallets.
Pallets that measure 48 by 48 inches are generally used to ship drums of liquids. Only 24 pallets can fit in a 48-foot trailer.