What NFPA code is used for stadiums?

A stadium is considered an "assembly occupancy" under NFPA 101 Life Safety Code and NFPA 1 National Fire Code.

Depending upon your particular issues, there would be cross-referenced standards that are also applicable to components such as sprinkler systems, fire extinguishers, construction types, use of LP-powered vehicles inside, etc, not to mention National Electrical Code (NFPA 70) and possibly dozens of others.