I was so baffled by three ranks. It sounds like that would be about 150 pikemen... quadruple it and we'd get 600 for the full army. Add in about a hundred archers per group, a similar number of cavalry, and the mixed group about equal to pikes: 150+100+100+150, so ~2,000. Pretty certain the knights are not meant to be larger than the Praetorian Guard. xD