The number of passengers is not a standard system in the sim, but something that airplane developers simulate in many different ways, either from external loaders, or with internal simulations, which might be progressive or not, there's no standard.
So, as explained on the GSX manual (Page 11), GSX estimates the number of passengers by dividing the total station load by 220 lbs, which is the industry standard for an average weight of pax+luggage, and this has always been used to calculate the passengers boarding/deboarding times.
While this might work well (although it might never be exactly the same, unless the 3rd party airplane has used exactly the same 220 lbs calculation) with planes with a full passenger configuration, those with a "combi" configuration won't be very accurate. We might further improve this, with an eventual GSX expansion dedicated to Cargo operations, which will have more complex airplane configuration parameters, indicating exactly which stations are passengers and which are cargo, how many ULDs can be loaded, etc.
Unfortunately, the station load system in FSX/P3D is quite primitive, and there's no such thing as a station "type", so we only know where a station is (for weight and balance), how much weights, and its name, but there's no sure flag to indicate what that station it's used for so, the best we could come up with, was adding all the stations together, and divide by 220 lbs, to give a rough estimate on the number of passengers. It was never thought as an accurate simulation, just to have a noticeable difference in waiting times between large and small airplanes.
Nothing has changed with this version, we just print out the result, to give some feedback of the operation in progress.