My recollection is that 12K is maximum legal load on a 80K semi and trailer combination; 10,280 was the maximum under the old (pre- STAA) limit of 73,280. It is possible to spec a lot higher, but of little worth, generally, if the rest of the weight is properly balanced. You may need to have a tandem, or least a rear tag, and move the weight back.
There are other factors, for example a maximum load of 600 pounds per inch of tire width; no more than 3' overhang past front axle (or bumper if there is one); no longer than 15' past the center of the rearmost axle (I got these out of my Washington statute book, but you can be sure that these are from the model code).