I'll recommend one wow folder.
And to double click the wow.exe multiple times.

The advantage of multiple folders, is different video or config options per warcraft.
But then you want to look at Symbolic Links and such...
If you do, this program makes them a lot easier: http://schinagl.priv.at/nt/hardlinks...kshellext.html
Personally, I'll recommend ISBoxer, as it virtualizes the config file, meaning one folder but the benefits of a folder per toon.

You can get free trial accounts (which cannot level past 10th), from the account management.
That way, you can see how many accounts your system can handle smoothly at once.
Dalaran (and Shattrath to a lesser extent) and Wintergrasp or Raids/BG's will run less smoothly then the starting zones.

If you are getting into boxing, with one PC, you'll want to look at software boxing options.

HotKeyNet and Octopus are free options.
And HKN has a ton of advocates on this site to give assistance.

Keyclone is $20 per computer, for a lifetime license.
It's a very strong option for keyboard broadcasting.
There is a wizard to assist your setup.

I've not tried GameCommanderPro.
But know its a pay option of some soft.

InnerSpace is $36 per year, and can work on up to 5 computers at once for the same fee.
It too has a wizard to assist your set up.
IS can be used for several games, in addition to wow.
It is a very strong keyboard broadcasting option.
With Keymaps which blow away simple broadcasting - you can have keys which only affect the slaves (arrow keys for movement) and keys which go to the healer only etc.
It also has instant PiP (picture in picture) swap speed, which is amazing and extremely useful.
It also has mouse broadcasting (HKN can too), which allows for ground target spells, talent selection on multiple <insert class>'s at once.
You can do repeater regions, for mouse click healing from any toon for the party/raid.
In my opinion, by far the best software boxing option around today.