Short answer: 2 - 10 homes
Long answer: Read on...
One of the key persons in a Neighborhood Watch is the Block Captain. They are the ones with the boots on the ground. This person is an integral component in keeping the block actively involved in crime prevention efforts. Many Neighborhood Watch groups flourish, but some dwindle, while others disappear totally. How successful they are, depends on the Block Captain and the support he or she receives from neighbors. You may not need more than one block captain if your neighborhood watch area is only a few homes.
How large of an area does a neighborhood watch or block captain have to cover? This map shows that you can cover 2 to 10 homes. It is entirely up to you. Do not ask anyone to overextend themselves or be overburdened. Pick the number of homes you want to represent as a block captain. Please volunteer today to be a Block Captain. More than one is welcome on any street.
Keeping an open line of communication with the neighbors helps built the Neighborhood Watch. Once formed meet and talk about expansion and see if your neighbors want to help organize nearby neighbors to join you. Expanding means the block captains would form a board to organize the entire area. Everyone in the area needs to be informed at all times by their block captains what their nearby neighbors are doing for them and vice versa. It is no easy task to cover a large area especially if there is no cooperation between the neighbors. We have gone full circle. Neighborhood watch works best when the neighbors know one another and are willing to help make a better place where you live.
If there is an existing neighborhood watch that has failed or failing in your area there is no harm in forming your own around your home. It is about neighbors watching out for other neighbors. You can try to breathe new life into the old group but look into why they failed in the first place. Sometimes a fresh start is better. Many fail because they took too big of a bite trying to organize entire regions rather than just a few homes.