What is the minimum square footage per person in an aerobics class room for 20 people.?

A group of 2 or a group of 1000? Not enough information is provided to answer the question unless you meant "per person". This isn't included in the question and unfortunately for the asker, but actually very fortunately, in math, what isn't stated can't be assumed. Secondly, what kind of group exercise? Again, lack of information.