Of course there are many different types of construction, but assume average since generally buildings have standard compliance codes and construction companies will do at least the minimum required. Of course, standards can vary across cities and across states and boundaries, but this post is not intended to receive a precise answer but rather something general.
Assume that a house or building is built to code of generally average residential requirements. Nothing extremely strong and nothing unreasonably weak, just an average room in an average residential building built to code, upstairs.
Moreover, people themselves vary a lot in weight so just assume an average human weight of 150 lbs.
A 300 pound refrigerator which is 33" x 36" = 36 pounds per square foot. This is obviously not a problem for any upstairs and there are also other things in the same kitchen including multiple humans, stove, cabinets, even washer and dryer are often in the kitchen and these are all no problem.
Extrapolating even this 36 lbs per square foot to a 12' x 12' room which is 144 sq ft, that is a weight of 5,184 lbs and may not be close to the limit, and this is exactly equivalent per sq ft to a standard 300lb fridge.
5,184 lbs is the equivalent of 34.56 humans weighing an average of 150 lbs each.
It is my guess that there is not enough square footage for enough humans to even stand in any normal sized room to come anywhere close to the weight limit for any room, even if you packed people in so tightly that you couldn't fit a piece of paper in between any two people. Maybe even if you also stacked the two high, which is more than can fit since with a rough average 5'5 height (averaging male and female), you cannot stack people two-high in a standard room which will be usually between 7 and 10 feet tall.
I wonder if you can even possibly fit enough people in a standard sized room in any way possible to come anywhere close to the weight limit for a standard room built to code.
I actually found a calculator online, it is on the Hilton website for Hilton hotels: https://hiltonworldwide.hilton.com/en/hotels/groups/popup_space_calculator.jhtml
In that calculator, if I put a room size of 144 (assuming sq ft), it returns a total number of people recommended as 12 people. Of course, a hotel is going to realize people are generally going to be a little crazy and do a lot more, so I am sure their estimate is quite a bit lower than the actual possible amount.
Their low estimate may also take into account fire regulations, of which aren't relevant to the question as to the maximum number of people that a room (of 144 sqft) will actually hold.
I found another site which said that the building code weight limit for residential homes on the west coast used to be 40 lbs per sq ft. This is the building code, which means that it is not the full limit, and that it was deemed relatively safe.
At 40 lbs per square foot which is pretty safe but not at the total limit, this would be 144 sqft * 40 lbs/sqft = 5,760 lbs, which would be 38.4 people averaging 150 lbs each.
So, based on the former code in that area of 40 lbs, nearly 40 people in a 12' x 12' room is not even the maximum number of people.
According to that post the new code is 30 lbs/sqft, which it is possible that the contractors use lesser materials than in the past to save money. In that case, a newer building with a code of 30lbs per sqft, even if the contractor did exactly to code and not a pound more, in that case it is 144 x 30 = 4,320 lbs / 150lb ppl = 28.8, so about 30 people.
Best Answer
Design is based on “working stress” and “ultimate stress”.
Working stress design accounts for many factors, including repetitive loading, short duration loading, etc. So, when the Building Code requires 40 lbs. per square foot LIVE LOAD, it is also adding in DEAD LOAD (weight of flooring, etc.,) short duration loading (Live load for less than 7 days), repetitive joist construction (single loads are rated lower).
Ultimate stress design calculates the maximum load (for live and dead load combined) before failure. Often this method is used, but then a “factor of safety” is used (such as 2) so there isn’t a failure.
You’re asking for the maximum number of people, but not accounting for length of stay, dead load, etc.
Also, are these people standing on the furniture, no furniture in room, hips touching, etc.? The code assumes a certain amount of furniture, circulation, etc.
Btw, at 40 psf, I’d assume about 36 MAXIMUM people. That’s 144 sf / 4 = 36. But they’d be in the room for less than 1 hour...depending on the species and grade of your lumber and size (thickness) of you subfloor, etc.