The Hydra event is currently the end of the demo—there's not much after that in this version, just continuing to play. No more buildings, unfortunately; they're coming though!
For the fight, you need at least 10 adult dragons to start that. Otherwise, I'd personally say you should have at least 15, with soldiers, for the fight itself. Most of your defense is gonna come from the buildings you put up after starting the raid. =)
Goodness gracious, that's a lot of dragons. I think I misunderstood your question initially; let me try again.
Most of your defense against the Hydra comes from building defensive structures; given how time-intensive most of them are, that's a bit tough. Dragons not assigned to be a soldier don't participate in the raid; they're sheltering and letting the military handle matters. Perhaps that explains it?