Gwaihir the Windlord
Actually until the musket became the main weapon of armies 20,000 men was a finr and regular amount for an army of a country with a population of probably about 1 million. Remember 20,000 Romans conquered Britain from Dover to Edinburgh and Julius Agricola believed that 5,000 legionaries and 3-4,000 auxiliaries would have been enough to conquer Ireland at the time. Rohan could be compared to England in the early 11th century and then Harold Godwinson would have raised an army of little over 10,000 then.