The correlation between the population of a country and its number of professional teams is rather a weak one.
A little more than seven months out from the 2019 Women's World Cup in France, the USA is on top of its game.
You have to go all the way back to 1994 to find an easier path the USA had to the Women's World Cup.
The Laws of the Game allow three types of substitution. In American soccer's ranks, though there are variations on the standard rules.