Malcolm didn't offer over 20x power on their scopes, so that would be the maximum used. A good number of guns leaving the factory with scopes never saw use as buffalo guns. That's is obvious from the number seen with scopes today in wonderful condition. Guns actually used by buffalo hunters show their use, and most have little finish left, even when well cared for. Metal and bore are often in great shape, as the owners considered their guns a tool and took care of them. But being out in the elements daily took it's toll and finishes suffered.
I don't doubt some buffalo hunters used scopes, but I do doubt that the numbers or percentages of scopes used is very high. I also don't doubt the military used scopes as there's a lot of scoped rifles pictured in photos of Berdan's Sharpshooters found. I would guess that the military continued to make use of scopes after the Civil War for either game harvesting, or possibly sniper use. Although Berdan's Sharpshooters group was disbanded before the Civil War ended, some of those men and their guns stayed in the military and possibly made it out West.
If anyone has some specific documentation of numbers of scopes used by buffalo hunters, I'd like to see that. Not just numbers of scoped guns sold, as that says little to how they were used. Scopes were very popular after their introduction by target shooters, in competition. So the requests for scoped guns continued to grow in the late 1800's and even more so after the end of the buffalo hunt era.
And when Sellers mentioned that 1/4 of the Sharps shipped Westward in 1876 had scopes, it's a bit misleading. Considering the location of the factory, almost EVERY Sharps leaving the factory went West to some degree. Tough to not go West from the East Coast. But considering there were supposedly less than a few hundred buffalo left by the 1880's, the majority of those scoped guns likely saw little use by buffalo hunters.