Rome had huge armies.
“This is a common misconception in the public, that the ‘legion’ was essentially the Roman word for ‘army.’ It wasn’t (that word was exercitus); the legion was, by the Middle Republic, almost always a component of a larger army. Indeed the standard Roman (consular) army was two legions (each 4800 strong), plus two equivalent allied detachments (alae or wings because they formed on the flanks) of equal size; a total force of notionally 19,200. Armies could be larger (and by the Late Republic, the allies are no longer deployed in alae, but now having Roman citizenship are recruited directly into the legions); Caesar had 11 legions in Gaul by the end, for instance. Indeed, a single legion was the standard size of a foraging party for the larger army”