Answer:
It was formed by eleven teams in 1920 as the American Professional Football Association (the league changed the name to American Professional Football League in 1921 and then settled on its current name in 1922).
The above is correct except that there were 14 teams in the APFA in 1920:
Akron Pros
Decatur Staleys
Buffalo All-Americans
Racine Cardinals
Rock Island Independents
Dayton Triangles
Rochester Jeffersons
Canton Bulldogs
Detroit Heralds
Cleveland Tigers
Chicago Tigers
Hammond Pros
Columbia Panhandles
Muncie Flyers