WoW's Cataclysm launch events kicked off last week with a pair of new stories explaining how the gnomes and the trolls finally get their hometowns back from the low level mobs that have been occupying them for six years. The beta has new introductory voiceovers for all of the existing races, and presumably other changes like these events will mean a lot of new low level content. My guess is that you're going to need anywhere from 2-4 new alts to see the 20-60 leveling content, and as many as 10-12 new alts if there are significant changes to the 1-20 content. To the best of my knowledge, WoW's per server character limit will remain at 10.
There are sound technical reasons for character limits - data storage is not entirely free for one thing. Even so, there is a distinct possibility that Cataclysm will test the limits of character slots per server on a scale that has never happened before in an MMO. The stakes are non-trivial; many longtime players have high level alts occupying all of their slots - some folks have all 10 level 80's. If you're forced to move to another server, you leave behind heirlooms (I'll have over a dozen when I decide what to spend my existing currency balances on) and social ties to your existing guild (which will also lose out on guild exp for your new alts).
I suppose there is a bit of a silver lining opportunity here. All of us probably know someone who plays on another server, and perhaps the Cataclysm is the time to pay them a visit. It's just going to be interesting to see whether we're going to see disruptive player migration when the new content meets the old character cap.
Bonus event commentary: "da druids, we been layin' low". Yes, we've seen various troll mobs that can shapeshift, but they have always referred to themselves as Priests of a specific animal loa. Until now, I can't remember a Troll who has actually called themselves a druid, or displayed the ability to change into more than one different kind of creature. This is why I don't take lore seriously as an argument to justify game design decisions - it's a fictional construct that can and will break as early and often as the developers want it to.