IMO unless you have/devote huge amounts of time to the game, if you are at least semi-serious about progress, you can only have one, perhaps two "real" toons. I'm talking about normal, progression, raids, not 12 months after the release of the last raid and at the end of an expansion. Perhaps LFR, even with its many, ok just one - people, shortcomings could make room for a further toon,
This is coming from my own experience and I'm sure more hardcore players will disagree, but on a five-day a week raiding schedule you're already putting at least 18 hours a week into the raid. Add your HC runs, LFR and of course the dailies and you are looking at about 35 or so hours at least (more at the beginning of an expansion). Every week. Now I imagine you won't be doing dailies on your alts (I mostly do, just fewer of them, on my "active" alts) and with the VP and JP changes to alts coming up in MoP you could shorten that to 25 hours a week per extra alt at level cap with all relevant professions maxxed (to me, cooking and fishing are relevant on your main as you need them to contribute/deliver fishing feasts). From there, it's up to the player, I guess.
Of course, top guilds will mostly skip HCs I suspect as they will be getting epic gear way faster and thus have no need to blast into HC territory, though they will still want to experience/see the instances and with the introduction of Challenge modes they might even be doing said HCs more come MoP.
Me? I had 8 80s at the end of Wotlk, but only raided on three, and only really raided on one. I've mostly skipped Cata and now I'm simply leveling said alts to 85, and two more toons I've rolled, at a leisurely pace. Come MoP I'll focus on my main and take it from there I guess.
Edited, Aug 27th 2012 10:18am by LGarth