In the beginning, and throughout most of Roman history, there were 6000 men in a legion. However, most of the military information and customs we think of as "Roman" came around Caesar's time, at which point a legion was comprised of 3600 men. In Rome's later days, legions were limited to 1,000 people to avoid giving commanders too much power.
I was just working with that on Monday night with Deborah. What brings up the question?