"The Indian Agent at Fort Berthold, C.W. Darling, stated that [Sitting Bull's winter village] strength was then believed to be about 500 lodges, and this appears to have been what Terry believed." - cited in a Congressional document in Taunton's Army Failures Against the Sioux in 1876 (1963)
That was in about February of 1876, when Terry was determining whether to mount a winter campaign against Sitting Bull's village on the Little Missouri river.
No matter how good you are, you have to get there first...