For horse-holders who can lead the horses around, from the saddle, 1-in-4 or even 1-in-3 seems typical.
Managing a bunch of horses that never move might be possible for a 1-in-10 horseholder, I dunno.
http://books.google.com/books?id=Rmg...ers%22&f=false
http://theminiaturespage.com/boards/msg.mv?id=253373
I seem to recall that the late 19th century Imperial Russian cavalry used 1-in-3; that is, one mounted guy, with an un-manned horse on each side of him. I think the Osprey book on the Russo-Turkish War mentions this.
As for mounted used of weapons: certainly pistol and sabre were the weapons of the U.S. Cavalry when mounted, in the early 20th Century. That's why each cavalryman was issued a pistol. I'm not sure if mounted rifle marksmanship was ever even discussed. The 1944 manual seems to presume that rifle usage will only happen dismounted, but I haven't read every page.
http://www.ibiblio.org/hyperwar/USA/...PDFs/FM2-5.PDF
It's for an eight-man squad. Ah ha, a command on page 38: "To leave horses immobile": "All horses of one squad may be linked in a circle and left to the care of one horseholder". Of course, if you had 10-man squads, I imagine one guy could probably handle 10 horses.
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Michael B.