The legio had ten cohorts. The Roman legion was divided into 10 cohorts: The First Cohort had more soldiers than the other cohorts. At the time of Julius Caesar, there were 600 people in the unit.

The expanding early Roman Republic found the Greek phalanx formation too
"What does a centurion do?" There were 66 centuries in a legion. Quantitative equivalent of the modern battalion. Cohort (cohors; plural cohortes) was a tactical unit of the Roman army with 3 maniple, so 6 centuries. The term legion also denotes the military system by which imperial Rome conquered and ruled the ancient world. The first cohort was made up of 800 men (5 centuries with 160 men each). 10 cohorts = a legion = 4800 men. We need you to answer this question! Legion, a military organization, originally the largest permanent organization in the armies of ancient Rome. The other 9 Cohorts had a total of 4,320 soldiers (6 centuries of 80 men each). "What is a cohort?" These questions are answered in this overview of the structure of the Roman Army at the time of the Judaean-Roman War of 66-70 CE. How many cohorts in a roman legion? Although 6 centuries made up a cohort and there were ten cohorts in the legion, the first cohort was double and had 12 centuries rather than 6. Roth says the Historia Augusta , an unreliable historical source from the late 4th century A.D., may be right in its figure of 5000 for imperial legion size, which works if you add the 200 cavalry figure to the product above of 4800 men.
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So how many men in a Roman legion? The Roman Army: Key Concepts. by G. J. Goldberg "How many men in a legion?"