I started with two accounts, across two computers.
Coming to WoW from an EQ1 background, where solo play was dead pretty much halfway into the Kunark (1st expansion).

Did not really need two toons, but had a healer type on autofollow.
And a main to play with.
Grouped the first two pairs of toons.
A - Warlock and B - Priest.
A - Priest and B - Rogue.
After that, had an ungrouped Priest on autofollow (at 60th) behind whatever I was "solo" playing.
No software, no broadcasting, but two monitors, two keyboards, two mice etc.

About a year ago, having seen an Aelli video, I came to this site.
Purchased a Keyclone license and a third account (RAF from A).

A few days later, did two more accounts.
A - C - D - E... with B for boosting.

It's been over a year now...
Not likely to quit anytime soon.
I'm certainly working on enough sets of characters.

When WotLK came out, I bought my 5 expansions.
The people in the store were asking if I was buying for friends or something.
And I said, no I play 5 toons at once.
They were like... how the hell do you do that?
I mentioned this site.