Payload capacity is your limiter. With nothing in the truck but a skinny driver, you can go by the payload sticker on the driver's door frame. Subtract around 200 pounds from that sticker and the remainder is your available payload for sheetrock.
For example, my F-150 has a max payload of 1,366 pounds, so with me and a little junk in the truck my max payload is around 1,100 pounds. If sheetrock weighs 50 pounds per sheet, that's 22 sheets max. I'd probably round it down to around 20 sheets max.
So take a bathroom scale with you to the lumber yard. Weigh one sheet of the exact sheetrock you plan to buy. You'll need to balance the sheetrock on edge on the scale without adding any weight to the scale other than the balanced sheet. Then buy a few less sheets than the remaining payload of your truck.