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.
Grumpy ole man with over 50 years towing experience. 2012 F-150 Lariat SuperCrew EcoBoost, 6.5' bed, Platinum White/tan, Leer 100XQ camper shell, Nomad Joey 196S TT, CarMate 7x14' enclosed cargo trailer, three other utility trailers, plus a retired 2000 Keystone Sprinter 25RKS 5er we rarely tow now. ProPride hitch on the TT. Reese Strait-Line hitch on the cargo trailer.