Life in the Roman Legions
In the early Roman Kingdom “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. During much of the republican era, a legion was divided into three lines of ten maniples. In the late republic and much of the imperial period (from about 100 BC), a legion was divided into ten cohorts, each of six (or five) centuries. 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. In terms of organisation and function, the republican era legion may have been influenced by the ancient Greek and Macedonian phalanx.
Credit to : TopTenz