Under NFPA code how many square foot per patron is allowed in a room?

The answer to this would be determined by numerous factors, primarily the type of occupancy and whether the AHJ requires further restrictions.

For example, National Fire Code, NFPA 1, requires a minimum of 30 square feet per person on the ground floor of a retail shop, but only 15 square feet of net open space per patron of an assembly occupancy, such as a dance hall, having no fixed seating, and 50 sq ft per person in a skating rink.