Although this probably isn’t a surprise to anyone who levels a lot of alts, it seems to me that the more characters one has at maximum level at the end of any given expansion, the easier it is to get more characters to maximum level in the next expansion.
At the end of Burning Crusade, I had one level 70 character: my Shaman, Kamalia.
At the end of Wrath of the Lich King, I had three level 80 characters: Shaman, Druid, and Death Knight.
At the end of Cataclysm, I had six level 85 characters: Shaman, Druid, Death Knight, Hunter, Human Mage, and Second Shaman.
Now, in the last raid patch of Mists of Pandaria, I currently have eight level 90 characters: Shaman, Monk, Warrior, Death Knight, Hunter, Druid, Paladin, and Second Shaman.
I will certainly have nine level 90 characters by the time the next expansion launches; when the Paladin reaches Exalted with the Shado-Pan (in a few days), the Priest will push to 90.
It’s quite likely that I will have ten level 90 characters by the time the next expansion launches; when I finish up the things I want to do with the Priest in Pandaria, I’ll probably finish leveling the Blood Elf Mage.
Depending on how long it is until the next expansion launches, I might even have eleven level 90 characters. The Double Agent achievement keeps nagging at me, and I’ve mentally flagged my level 27 Dwarf Shaman as the character I want to do it with.
Will there be more than that? I don’t know. I’m starting to feel somewhat weary of the Pandarian rep grinds, so further alts may have to wait until sometime in the middle of the next expansion, when returning to Pandaria will be a welcome, refreshing respite from that expansion’s content.