A Roman legion (from Latin legio "military levy, conscription", from legere "to choose") was the largest unit of the Roman army involving from 3000 men in early times to over 5200 men in imperial times, consisting of centuries as the basic units. Until the middle of the first century, 10 cohorts (about 5,000 men) made up a Roman Legion. This was later changed to nine cohorts of standard size (with 6 centuries at 80 men each) and one cohort, the first cohort, of double strength (5 double-strength centuries with 160 men each).
In the early Roman Kingdom the "legion" may have meant the entire Roman army but sources on this period are few and unreliable. The subsequent organization of legions varied greatly over time but legions were typically composed of around five thousand soldiers, divided during the republican era into three lines of ten maniples, and from about 100 BC into ten cohorts. Legions also included a small ala or cavalry unit. By the third century AD, the legion was a much smaller unit of about 1,000 to 1,500 men, and there were more of them. In the fourth century AD, East Roman border guard legions (limitanei) may have become even smaller.
For most of the Roman Imperial period, the legions formed the Roman army's elite heavy infantry, recruited exclusively from Roman citizens, while the remainder of the army consisted of auxiliaries, who provided additional infantry and the vast majority of the Roman army's cavalry. (Provincials who aspired to citizenship gained it when honourably discharged from the auxiliaries). The Roman army, for most of the Imperial period, consisted mostly of auxiliaries rather than legions. :) hope this helps you out
