For EMT (anyway) the spacing is pretty clear - I can't recall if it's the same or different for other types, but it might well be the same.
Within 12" of each box. 10 feet maximum spacing (1 clamp per stick of conduit, minimum.) You can do more than the minimums without an issue. You can't do less than that. So a typical 20 foot run would need at least 3 (both ends and the middle), or go wild and use 4.
Despite being called "hangers" a random manufacturer or dealer page I visited indicated that the device was "suitable for horizontal or vertical installation." To be persnickity you would verify that the manufacturer of the specific device you purchase says the same, (so that you are "using it according to its instructions or labelling") but that's likely universal.
I would use short hot-galvanized or stainless steel lag bolts. Length should be such that they don't go more than 1-1/4" into the studs, and I would put them on studs, by preference, rather than just siding, and I'd spend the dime or so on a washer as well. You could probably get away with less, if you wanted to. I prefer not to have to revisit work later when it rusts away or pulls out.