USSF
F.I.F.A
The top flight is MLS, Major League Soccer.
By pro I guess you mean MLS which is the first tier of American soccer, in that case it's 15 teams.
David Beckham
pro soccer players
There isn't really one acronym of soccer. There are multiple. For example, MLS means Major League Soccer. There are many more acronyms for soccer, but I know only one.
You can be as young as 15 to join a pro soccer team
Soccer
AWS Pro Moves Soccer was created in 1993.
AWS Pro Moves Soccer happened in 1993.
International Superstar Soccer Pro happened in 1997.
Pro Evolution Soccer Management happened in 2006.