Thirty-eight million American adults — 15 percent of the U.S. adult population — are planning to bet on NFL games this season, according to a new survey by the American Gaming Association. The rapid expansion of legal sports betting has provided increased opportunity for millions of Americans to bet safely with a licensed, regulated sportsbook. This year, 7 million Americans say they will bet legally at a casino sportsbook, 1.2 million more than last year, and tens of millions of others will place bets with friends, participate in pools or squares contests or place their bets online. “During this centennial…