I only have one level 90 (my main character) right now in MoP. WotLK, I had three level 80s that I raided with. Cataclysm, I had six level 85s but I consider this an outlier because old-world flying allowed me to easily level characters through gathering professions - and at least three of those 85s were leveled solely for guild Classy! achievements.
I will likely finish leveling my Alliance character to 90 for the Double Agent achievement. Other than that, I don't foresee having additional level 90s for MoP, nor ever having more than one max-level character if future expansions continue Cata/MoP trend.