Just one. I leveled up a warrior to 80 first (during WotLK), then a priest. The priest is my main until today.
I went through only one character before my main. I started in BC and got a warlock to lvl 69 before giving up and starting a Paladin, its been my main since then. I've made every class to 90 since then and raided with a few of them but my paladin is still my main.
My main has been a druid since late BC. Though I've had multiple druid chars. This expansion it was mainly a hunter, though I've had 3 rogues (so much fun to level them up).
I usually like to stick with one or two chars (Druid/Priest usually but lately just been wanting a full DPS class so Hunter/Rogue seems good)
The very first toon I made once servers went live on launch day was a mage named Harry Botter. I then moved servers to Ysondre in 2006 when I learned a few friends at work played too and that was their server. So I just remade the same toon. It really sucked leaving a toon that had almost full epics to nothing.
1st was a hunter.
2nd a warrior.
3rd a paladin(main).
I think i'm gong either Shaman, Warrior, or Warlock for WoD though, if i decide to play it.
The second character I ever made when I started playing in the beginning of Wrath is still my main today. A hunter.
Started as a Hunter in 07'. Switched to Druid in Cata. Back to hunter in MoP. I guess my hunter is my main.
Took about 4 characters. Mage, shaman, priest and then finally warlock. Always been a caster dps at heart, just took some time to find the one I was after.
Depends on what you consider your main. If its just the character you raid on mine keeps changing based on the raids needs, switching guild etc.
If its the character you play most, do achievements on and attempt to raid on every expansion even if its just an alt raid or pug, then it was my first character which is my Warlock.
Raid wise I did the most on my Priest and that was my second max level character, next would be my DK. (if you can't tell early on I was dps, then I was a healer, then a healer/tank, then a tank, and now I'm back to just dps.) In WoD I might be returning to healing based on the guilds needs, sadly it won't be on my priest as we have three and two have been dedicated members for years. I look forward to possibly being a druid though.
OT: depending on meaning either first guy or still alternating them around.
First character ever was a human warrior back in vanilla. Got bored with that at about level 10, then switched to an orc hunter and never looked back. :-)
i started late ICC so meh...
Tier 10: DK DPS
-Cata-
Tier 11+12: DK DPS
Tier 13: Hunter
-MoP-
Tier 14: Hunter
TIer 15+16: Balance Druid
-WoD-
Tier 17: No clue (im thinking tree druid)
PvP
Seem to change each season really starting with MoP
Balance > Ret Pally > FDK > UH DK
Pretty much this, except I've been pretty solid about staying with my Warlock and Paladin over the years. Warlock just RECENTLY got new mechanics, although my favorite specialization (Affliction) still plays very similar to years past. Paladin changes every expansion almost so that never gets old in changing play styles, but it does get annoying. However I can't decide in the end, I've made 100+ characters over the years and I'm sure at least 25% - 50% of them have been leveled to maximum at their respective expansions.
However I finally decided in Cataclysm and MoP to just stop that and kind of have 11 characters I play now. Or is it 10? Whichever the class amount is.
I pick one main for every expansion. Only exception so far was Mists, in which I had three (one until 5.2, another until 5.4, and another afterwards).
Yak. Yak never changes.
Vanilla started as a Warlock, finished as a Rogue
TBC started as a Rogue, finished as a Paladin
Wrath started as a DK finished as a Paladin
Cata started as a Rogue finished as a Paladin
MOP started as a Monk finished as a Paladin
I've played every class at max level, but those were my "mains".
I keep getting fed up of the Paladin (gameplay wise) but then keep going back due to rep/achievements/aesthetics and Lore... learning my lesson for WOD, just going to put up with whatever state the class is from day 1. So sick of main changing mid-expansion and re-doing everything on the Pally. I've come to realize that if you swap mains everytime Blizzard messes with your class, then you end up swapping every expansion (if not every patch!). May aswell just roll with the punches.
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Four.
Resto druid healing in vanilla. Wasn't fun once I started raiding.
Shadow priest through bc and wrath. Loved it, but dps got old.
Guardian through 95% of Cata. Tanking is win, but all my friends are good tanks for some reason, so I switched to...
Resto Shaman. Love healing. Love this spec. Now my main for an entire expansion.
I started with a paladin in Wotlk when i first started but swiftly moved onto a shaman, played that for a few years swapping between a mage also. Played a DK tank for part of cata also. Early mists i rerolled into resto druid and honestly can say i've found my 'main' class now.
It took me 7 years to find out I'm a rogue by nature. Was one of the last classes to try out by then.