The NFL Commissioner makes $10 million per year, so they at least generate, somehow, $10 million in revenue.
Whatever "revenue" they have is only to pay their expenses, including the cost of office space, salaries for their staff (not just the commissioner but his administrative staff), etc. That's how a non-profit organization works.
The NFL doesn't get any revenue from its football operations. My guess is that all of their income is from fees that are paid by each franchise.