How much weight/many people can my rooftop deck hold? 2“x12”x20' joists, 16“ on center, 2”x4" framing under
The allowable load on the deck depends on the species and grade of your 2x12’s. 

You said the deck was “approved and inspected” and it met “Code”. Therefore, I’m assuming the joists at 16” o.c. are Hem-Fir No. 1 grade or better. In addition, I’m assuming you have at least 1 1/2” bearing at each end and there is solid blocking (or adequate joist hangers to hold the 2x12’s in line) in order to meet Code. Because you called it a deck, I’m assuming it’s outside and is pressure treated (incised). (Incising reduces the strength.)

Joists spanning 20’ at 16” o.c. can support a “Live Load” (people and furniture, etc.) of 30 lbs. per square foot plus a “Dead Load” (framing, decking, etc.) of 10 lbs. per square foot. 

30 lbs. per square foot is slightly below Code. (Code is 40 psf). If you use Grade: Select Structural, it will meet the 40 psf.

You didn’t give the size of the deck, but you can multiply the area by 30 psf to give you the maximum load your deck will support. 

The Code does not stipulate the average weight of individuals, but you might use 180 lbs. as an average to determine number of people the deck could support.

The code allows a few increases, such as “short duration loading” and “impact loading”. I suppose both increases could be taken, but I don’t know enough about your deck to use them...so let’s just use that as a “safety factor” in case you start loading your deck up with kegs too.

If you’re thinking of loading your deck to the maximum, you’ll also want to calculate the required size of the footings, required size of joist’s supports (beams), posts fixes, required diagonal bracing for stability when they start singing “for he’s a jolly good fellow” at your birthday party...but that wasn’t your question. Party on...