How much weight can a diesel truck carry?

Not enough information to answer. What type, is it a straight truck or tractor trailer, how many axles, what class vehicle, etc. would be needed to give you an answer.

With weight laws in the US, a single drive axle truck rated at 26,000 lbs. gross vehicle weight could carry up to five tons, while a Class 7 single drive axle truck rated at 33,000 - 35,000 lbs. could carry up to eight tons. A tandem axle straight truck would be in the vicinity of 12 to 13 tons, and a standard, five axle, semi tractor and trailer can range from 23 to 25 tons of legal payload.