Write down all of the ratings and limits on a sheet of paper. The smallest one is your limit.
My F-150 has a hitch tow rating of 12,000#, truck towing rating of 11,800#, a payload rating of 1,895#, a hitch tongue weight rating (w/WD hitch) of 1,200#. My limiting factor is the payload of 1,895#.
1,895# minus 400# for the two of us minus 200# for the topper minus 100# for the WD hitch, leaves us 1,195# for tongue weight and "stuff." At a 13% tongue weight to trailer weight ratio, I can tow 9,200# if I don't carry other "stuff" in the truck. THAT is my tow limit.
On Edit: I want to address your bumper pull. If it truly is a bumper pull (and a lot of pickup trucks DO have a ball on the actual bumper), you can not put a WD hitch on it. It must be a frame mounted receiver for that. So, if it truly is a bumper mounted ball, you are limited to what it says on the bumper unless you go to a hitch shop and get a frame mounted hitch.