From the passage provided, extract the progression of number of games in an NFL season. 
 In the early years of the NFL the league simply mandated a minimum of 12 games played each season from 1920-1934. In 1937 teams played 11 games. In 1943 teams played 10 games due to World War II. By 1947 teams were playing 12 games per season. From 1961 to 1978 teams played 14 games per season before playing 16 games per season until 2021. Each NFL team plays 17 games per season in present day.