Explanation:

- In football, also known as soccer in some countries, each team can have a maximum of 11 players on the field during a match. This includes the goalkeeper.

- Option 1: 12 - This is incorrect. Having 12 players on the field would result in a penalty for too many players.

- Option 2: 9 - This is incorrect. Although a team may end up with 9 players due to red cards, the game starts with 11 players.

- Option 3: 10 - This is incorrect. Similar to above, a team may temporarily have 10 players.

- Option 4: 11 - This is correct. Each team begins with 11 players, including the goalkeeper.