We can get 12 adults comfortably in our 8' round hot tub....

but wider is better for kids jumping in etc....much safer in my mind.

I have a 16x75 and that would be crowded with 20 swimmers from a safety point of view. 6 feet does make a difference, but I wouldn't have 20 people in the pool without a dedicated/undistracted lifeguard.

As far as displacement goes, people will displace about their weight in water and water is about 8.3lbs/gallon (pure water. Salt water would be heavier). 150 lbs/avg person weight divided by 8.3 lbs/gal x 20 people = 362 gallons displaced for 20 people. With a 14x28 pool, the water will rise 1.5" when they enter.

Depends on how fat the people are.

Depends on what size keg you have at the party IMHO.