In most cases, the purpose of a wall is to make it easier on the keeper to protect the goal from a direct on indirect shot.  However it’s important for the keeper not to put so many people in the wall that it allows too many other options for the opponent.  For example, if there is an indirect kick from 35 yards out and the keeper chooses to put 5 players in the wall.  While this would most likely eliminate any possibility of a goal resulting from a direct shot on goal, it also will most likely leave a number of opponents wide open and in good goal scoring positions.  Ideally a keeper would have enough players in the wall to prevent goal from a direct shot on goal (by making the area the keeper has to protect smaller) while at the same time not putting too many players in the wall that there are many options for the opponents. 

Another risk of having too many people in a wall is that it makes it difficult for the keeper to both see the ball and also cover the near post.  Typically a keeper will line up in  a position so that the keeper can see the ball just past the last player in the wall (towards the far post).  This way the wall covers driven balls to the near post and the keeper covers driven balls to the far post.  However, if there are too many players in the wall, this would force the keeper too far toward that post which would make it difficult if not impossible to cover the near post on chips over the wall.  By having fewer players in the wall, it puts the keeper closer to the near post for these chips.  The key is to have enough players in the wall to protect driven balls near post while not so many as to expose too much of the goal to a driven shot. 

It’s difficult to say exactly how many players should be in a wall when the ball is in a specific area because it will depend upon the ability of the keeper, the ability of the player(s) on the ball, and the tendencies of the attacking team.  As a general rule, when the ball is around 18 yards from goal you can see a general idea of how many players should be in the wall in the diagram below.  The further out the ball is, the less players are needed in the wall.

Remember, use as many players in the wall as is necessary but not a single player more.