How many pounds can you carry into a semi trailer?

The weight that can be carried in a semi trailer legally varies. It depends on how much the tractor pulling the trailer weighs. The maximum weight of the total unit, truck and trailer is limited at 80,000 lbs without getting special permits.

For a heavy tractor, the trailer would have to be loaded with less weight than a trailer pulled with a light tractor.

It's not uncommon for a trailer to comfortably carry 45,000 with most over the road tractors. Some trailers can be loaded heavier as long as the don't exceed the maximum.

Also, I should mention, axle weight will affect how much can be loaded also. only 34,000 can be loaded on the trailer and tractor rear axles.

This means that the weight on the trailers must be properly distributed.