I've seen videos of a 2-boxer doing Kael'thas (with 23 other single boxers), and Ellay had a video of 4-boxing Vashj (with 21 other single boxers).

I don't believe anyone has ever attempted a full 25-man all-multibox'ed raid. For one, I don't believe most of us have 5+ raiding interested multiboxers of the appropriate gear level on the same server. <The Zerg> on Magtheridon has the numbers but I think they're more of PvP oriented?

Suvega and I can 2x 5-box Karazhan (all except Nightbane & Netherspite --we've never tried either of them), and I believe Nixi has 10-boxed similarly.