I’m thinking ten myself. Playing games like Age of Sigmar, and similar skirmish style games, five produces a minimal sized unit, so ten can create either two small cavalry units or one larger unit, and keep the price per box manageable. It also means the folks here can cram more options and spare parts on the sprues.
Gasmask equipped riders (and horses) could be neat (and creepy if a line of them were seen atop a cliff). Maybe kit them out in such a way to work as early WW-I, with options for use in Death Fields or other sci-fi cav (say as Rough Riders useable in older 40k rulesets)? For that matter, the actual Rough Riders would be an awesome set, especially if one could get an option to field ol’ Teddy Roosevelt himself.