412 by doing the math of 120 times 12 inches divided by 3.5 but I would buy more and return the ones you don't end up using.
Yes. Raw (cheap) untreated lumber. Yes they will eventually rot, just slap a new set together every 5 years. Get 3, cut 1 in half for the end pieces. 6x3 and easy to work with. Been using these for years with no problems.
I bought and installed a couple of these this summer, and they still look new, but I'm assuming they will eventually gray.
No. These pickets will shrink slightly and will leave a gap even if abutted at installation.
On average, 3 per foot (that's 9.5 inches of board, with spaces in between) However, if you leave less space (good luck, if the boards are not straight) or more space, it will vary. 240 would be average. Consider, as well, whether the boards are very wet (as usual) or dried. They will shrink and warp if wet, so your nice spacing will disappear. For nice projects I've gone to using Kiln Dried ones available elsewhere.
Truth? You bet! We are actually considering doing just that. It is real cedar and very easy to work with and if you want a smooth surface, this would go through a planer very easily. Plus it smells good and looks good.
They do vary but usually they are 6'.
No, they are rough sawn.
Avoid Thompson's Water Seal, unless you want to recoat every year! And then after two or three years, you will want to replace the wood! Use an oil based exterior paint, instead, or Flood Fence and Deck stain (the old kind, not the stuff they sell now, which is water based. Good luck finding it in old stock.)