The airplane's published range will be based on distance that can be flown in still air (excluding winds in other words) with full fuel, departing at max gross weight, when flown at some optimized cruise speed or mach#, at a specified flight level, with normal IFR fuel reserves (fuel to go from A to B, fly an approach, do a missed approach, proceed to alternate, fly an approach there, and land, with, for a jet, 30 minutes remaining after all that), in standard ISA atmospheric conditions.
Somewhere in the fine print in the airplane's specs will be the Mach# and altitude that the maximum range is based on.
Most airplanes can't be flown with full fuel AND a full cabin without being overweight, so you have to horse trade. A corporate jet with fuel for 5 hours endurance may only be able to carry 3/4 of the passenger complement to get max range, or conversely be limited to 4 hours if the cabin is full. Such a jet may have an option for a long range fuel tank add-on, increasing endurance to 6 or 7 hours, and if that tank is also full, the passenger load may be cut down to just a few passengers.