On the BBJ 8 that is...
"The BBJ MAX 8 may be fitted with up to eight auxiliary belly tanks, but seven or fewer tanks afford the most practical tradeoff between range and payload. Three forward and four aft belly tanks hold 24,455 lb. of fuel, for a total capacity of 70,370 lb. The aux tanks transfer fuel to the center tank using cabin pressure. While the wing and center tanks can be filled in 15 min., Boeing estimates that it will take another half hour to refuel the seven aux tanks."
BBJ MAX 8: It?s Bigger, Flies Farther And Is More Efficient | BCA content from Aviation Week