In most cases, no, they all won't be banned unless you're botting under all of them. Can they all get banned? Yes they can, but its not super likely.
All accounts under a b.net share almost all achievements, mounts and pets (the top end pvp achievements and mounts, no). It's good to have a second account to protect your main, but its not necessary. It may be good to have the account before a potential ban, but you'd most likely be ok even if you got another account after a ban.
Cheers, if I return to WoW I'll probably bot on a 2nd account to earn gold for my main which I wont bot on. Something along those lines anyway.