"A Roman legion (from Latin legio "military levy, conscription", from legere "to choose") normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but they were typically composed of up to 5,400 soldiers, originally divided into three lines of 10 maniples (see Roman Republican Army) and later into cohors (cohorts) each with around 480 soldiers. Maniples would usually consist of two centuriae which were originally at a strength of 60 men cohorts (with the exception of the Triarii which had half strength units) but with theMarius Reforms the Cohort replaced the Maniple as the main tactical unit and each Cohort was divided into six centuries of around 80 men each."

First paragraph of the Wikipedia article on Roman legions.