The Roman army at its largest likely numbered between 150,000 and 180,000 men in infantry and cavalry, not counting slaves and other civilian adjutants. The legions swelled as large as 6,000 and numbered 30 during the period of the Empire.
While Rome was still a republic, the size of the legions was smaller. Historians estimate the size of each legion as between 4,000 and 5,000. There were only 10 legions during the time of the Roman Republic, which means that the Roman shift to empire led to a dramatic increase in the size of the military. However, the barbarian tribes from northern Europe were able to fight ably against legions with numerical superiority through their use of early guerrilla warfare tactics.