Why 11 is the number of player on a football team?

It was a compromise between the different sets of rules which existed at the time. Until the mid-1800s football rules still varied across the regions. These rules were set by the English public schools and universities where the game initially developed. Team sizes ranged from 9 to 21. The 11-player team was standardised in 1870. It was probably influenced by the fact that a cricket team also has 11 players - cricket was the most popular team game in England at the time.

English public schools and universities, Cricket team, Most popular team game in England