A football team must consist of no more than eleven players (and a minimum of seven). When was this number chosen and is there any reason for it?
From this article Players in a football team we can get some hypotheses:
The big change arrived in 1870 with the agreement of the regulations that, amongst other things, established that there should only be one goalkeeper who stayed in a fixed position. Similarly it was decided that the number of players should be 11. The reason why this particular number was chosen is not exactly clear: it is believed that after many trials and tests the conclusion was reached that 20 players were enough to cover the pitch and to this it was necessary to add the two goalkeepers previously mentioned.
For other people the reason behind it is a lot simpler: the football team managers wanted their sport to be as popular as or more popular than the most famous at that time, cricket, so they copied the number of players.
It is indeed interesting to note that many sports have 11 players per team: American football, field hockey, cricket...