ISO (international standardization org.)recognizes six different size pallets as standard. You have to be more specific.

In the US , the GMA (Grocery Mfg. Ass)pallet is the most popular at 48" x 40 ''.
But there are others including the beverage pallet at 36" x 36".

Is this a trick question? If your trailer is 48 feet long, and 98" wide - and a standard pallet is 3' x 3':
lengthwise, 48 ft divided by 3 feet (per pallet) is 16. However, that would not allow for any overhang of the goods loaded on pallets. Also, if you wanted to put 2 rows of pallets, concievably you could put 32 pallets. And, if the weight and type of packaging on the pallets allowed, you might be able to double or triple stack them. If you are loading only pallets with no goods on them, you might be able to load 6 or more high.You have to figure the total weight of your load.You do the math.