Some obvious disclaimers:

• Only level 70 characters are included, so this will not necessarily reflect the realm as a whole
• Not all level 70 characters are counted. Realms with low population counts will have skewed and unreliable figures.
• Since there is a tendancy for guildmates to also be included, this effect can skew numbers greatly towards one side.
• Since this calculates by character and not by player, this effect can skew numbers greatly towards one side.
• This is in no way suggesting that the real overall realm counts are anywhere close to the figures presented below.

Realm counts for characters in the wowmetrics database. This page is updated daily:

Realm

Total

Alliance

Alliance Percent

Horde

Horde Percent