Hmmmmm.... good questions.
As far as how many people can a 20' screen accommodate? The max number I see most often for a 20' screen is 500. But, there's really a lot of variables to consider.
Here's a few off the top of my head:
How much seating room is there for your audience?
If you have a lot of seating room, obviously there's more room for people to sit. However, that's doesn't always lead to a good viewing experience. Smaller spaces force people closer together. As long as it doesn't force people to sit on top of each other, it can lead to a better viewing experience for a larger number of people. With a good balance of space, 500 people is definitely workable.How much of a "Movie Theater" experience do you want for all viewers?
Similar to the question above, if you want a "Big Screen" experience for all viewers, I would chop the number down to 250 and under.How well can you cover the audience with sound?
This is probably the most important thing to consider. Two 15" high powered speakers at an absolute minimum. But, I would go with four 15" speakers, and possibly a couple sub woofers. With the right sound coverage, 500 people should not be a problem. If you don't mind people spreading further away from the screen, you could bump this number even higher. Possibly 700? But, remember, this would most likely mean that the back of the audience could be as far as 200' (or more) form the screen. That's hardly going to be a "Big Screen" experience. But, as long as the audience can hear the movie with good sound, I'm pretty confident you could go well over 500.Can you handle the concessions?
Since you brought up concession, I'll mention it here also. Several hundred people watching a movie will get hungry, and thirsty. You'll want plenty of concessions, and be able to serve without creating lines. Popcorn and water bottles at a minimum. You can add soda, pretzels, Italian Ice, cotton candy, etc. also. For 500 people, I would have a pretty good selection, and at least 3-5 people dedicated to concessions.
Concessions can be a good money maker. The markup is great. The problem is over purchasing, and over making. It can be extremely difficult to predict demand. If the event is a fund raiser for a particular group, they should handle the concessions. If it's just a public event, then question becomes, do you own the concession equipment? And, how many events like this will you be doing? Meaning, does it pay to purchase equipment? If it's just a one-time thing, I'd probably pass on the concessions. The cost to rent the machines, and the hassle might not be worth it.
Again, all just my opinion, and right off the top of my head.