it depends on what formation you are playing but normally 3 or 4 at most.
A back is a defender.
Neelanjan LakshmanPique
Yes, if you are a defender.
defender
Pressure of the defender
Defender...
a defender that is on the right side
goalie, defender, midfielder, forward
Today it is the Brazilian Danny Alaves.
Defender or Midfielder
John Terry of Chelsea and England.
The defender usually depends on the sport. In soccer (football) the defender is the one that protects the goal. I don't know much about football (American Football)