How much cc to leave for passengers and gear?

Pretty general question, how large is your family, and do you have large pets. The best way to approach this issue is to load up your TV as you would for a trip including hitch and passengers. Then head to the scales and weigh each axle, add them together, and subtract from GVWR and that is what you have left for pin weight.

For the most part just the two of us not. Our combined weight is 350. With full fuel, both of us in truck and hitch in back, we were about 600lbs shy of our capacity after the unloaded pin weight. This also included a Blackstone griddle in the truck.
I'm thinking that leaves me enough extra weight in case we would have another couple with us. And, if we had extra gear stored in the hauler, I am assuming that would lessen the hitch weight?

You will find that adding more to the hauler really doesn't reduce the pin weight as much (if any) as you might expect. I'd suggest taking your setup across a set of scales. The best network of scales is to find the nearest CAT Scale and see for yourself. Taking it loaded and unloaded can be eye opening.