How many players from one team are on a football pitch?

Eleven, including the goalkeeper. The number was fixed in the first codified rules in the 1860s and appears to have come from English public schools, where a cricket eleven was the natural size for a team and the same boys played both games.

The goalkeeper was not a special case at first - the early laws let any player use their hands in their own half. The dedicated keeper appeared in 1871, and the restriction to the penalty area only in 1912. Before then a keeper could handle the ball anywhere in his own half.

The rule that shaped the modern game is the offside law, and it was originally far stricter: three defenders had to be between the attacker and the goal. Changing that to two in 1925 caused goals per game to jump immediately, and produced the tactical shape everything since has been a variation on. Football's history is largely a history of arguments about this one rule.