Before 2005-03-07 all dungeons were not limited at all
After that dungeons were capped to 10 (did Strat with 10 players)
UBRS was 15 (did it)
And other raids 40 of course
"Dungeon Player Caps

All dungeons now have a cap on group size to limit the number of players that may enter the dungeon. It was necessary to add this cap as raiding these dungeons trivialized much of the content. Because of this change we can now look to adding better loot to them during future content patches.

With that in mind, the dungeons below now have the following caps:

Onyxia/Molten Core - 40 members
Blackrock Spire - 15 members
Dire Maul - 5 members 

All other dungeons in the game will be capped at 10 members.Several dungeons will be re-examined for balancing in light of thischange. In addition, will have their loot improved significantly for the next content patch. "