How much freight can a Boeing 737-800 NG carry if the majority of seats are removed?
At 160 seats, a -800 carries 16 tons of people in the cabin, plus freight/luggage in the belly, plus fuel. At 180 seats, that becomes 18 tons of pax. If you assume one 50# suitcase per person, add a quarter of that again, so 20 to 22.5 tons.  Much more than that you're probably starting to run up against the Max Zero Fuel Weight limit, although I don't have the right charts in front of me to give an exact answer there.

If the flight is long enough, max takeoff weight becomes an issue, as your fuel load starts to cut into your lift. Obviously lots of factors go into how much fuel a flight requires; "today's" winds &amp; temperature &amp; destination weather (alternate required? How far away?) will change how much weight today's flight can take.

Ballparking it with back-of-the-envelope math, anything up to about 3-4 hours, you can probably get those weights I mentioned above (i.e. 22 tons or so). Much more than that, I'd expect max takeoff weight to limit you, so more distance = less payload.

If the legs were 5000km between refueling stops, then you're well into the limit being max takeoff weight, rather than what the jet could carry on a short-haul.

Depending on the nature of the cargo, the limits on floor loading in the cabin + the cargo compartments may be limiting, or volume may be limiting. If you're hauling pingpong balls, it's all about volume. If hauling gold, floor weight capacity &amp; total weight will be more limiting. If it's all rucksacks, then you can probably leave the seats in &amp; load around them, and only total weight will play.

Disclaimer, doing this with no references in front of me, and I may have a number off in there. When there is a specific need to haul "this" from A to B, the guys with the charts &amp; load plans come up with a far more specific answer. The above is simply a rough ballpark.