My copy of Amber Books' The Essential Aircraft Identification Guide: Aircraft of WWI gives numbers in August 1918 as being:
- France: 34% fighter, 51% observer, 15% bomber
- Britain: 55%, 23%, 22%
- Italy: 46%, 45%, 9%
- USA: 46.5%, 46.5%, 7%
- Germany: 42%, 50%, 8%
- Austria-Hungary: 63%, 28%, 9%
hope that helps!