How much weight can a single axle truck pull by law?

You're making up laws and asking about them. Typically, the max gross vehicle combination weight permitted for interstate travel is 80,000 lbs. A single axle tractor pulling a set of doubles can do this.. in states which permit triples, it can have an even higher gross vehicle weight rating.

It also depends on your class of licence. If you have a regular Class C or D (depending on the state issuing it), you can operate any combination of vehicles up to 26,000 lbs. Gross Vehicle Weight Rating. With a Class B CDL, you can operate a combination of over 26,000 lbs. GVWR, so long as the vehicle in tow is rated at and is weighing less than 10,000 lbs. And a Class A CDL is required for combinations rated over 26,000 lbs. GVWR when the vehicle in tow is rated at over 10,000 lbs.

So long as the total weight rating of the combination is within the range specified by the class of licence you possess, and you're compliant with axle weight laws, you're good to go, generally.