Complex sentence: During the First World War, the Sherwood Foresters had 33 battalions in service, of which 20 served overseas: mainly on the Western Front, but also in Gallipoli, Italy and the Middle East.
Split into simpler sentences: During the First World War the Sherwood Foresters raised 33 battalions, of which 20 served overseas. This was mainly on the Western Front, but also in Gallipoli, Italy, and the Middle East.