How much weight can a flatbed trailer truck haul?

Legally, in the US, it's typically between 43,000 and 48,000 lbs of payload. The max legal interstate weight is 80,000 lbs. for the truck, trailer, and payload. How much the actual payload comes out to depends on the empty weight of the vehicle and trailer, and that'll be affected by a number of factors, such as the frame length of the truck, what the trailer is made of (steel or aluminum), etc.