How many is a legion?

The Roman military force was called a legion.  As the military doctrine and capability of Rome evolved, so did the Legion.  At first, it was a name used to refer to the entire Roman army.  As the army became more organized, legion  came to mean a fighting force of five to six thousand members.  During the peak of Roman military might, the basic unit was a Centuria, an 80 man unit when at full strength.  Next was a Cohort, which had 6 Centuria plus a cavalry unit, usually 20 to forty members.  The Legion had about ten Cohorts, so it was about 5,400 troops.  Since "legion" always meant a large number, the word came to mean a group of people with many members.