I always start out with one class, then 1 or 2 months later i start getting bored and lvl 2 or 3 alts then switch between them throughout the expansion.
I'm playing a shaman atm, but im planning to lvl my warrior, paladin(my main from 08 until 3-4 months ago), and my druid. I usually play my warlock too, but it ended up being a waste of time in MoP since locks were incredibly boring, and they are even worse in WoD so ill just skip the warlock. I usually lvl up a mage as well but never actually do anything once i reach max lvl.