At its largest, the Roman army in Britain consisted of 50,000 well-trained troops. There were two main kinds of Roman soldier. The Legionaries were Roman citizens from across the Empire. The Auxiliaries were non-Romans who were recruited from other conquered provinces. They were not paid so much, but became citizens when they retired.
Despite such a large army, the Britons still managed to cause the Romans difficulties and they even overran Hadrian's Wall on one occasion.