Just my opinion...
But, you are never allowed to have more than 1 attack of opportunity against an opponent in 1 round no matter how many times he provokes and no matter how many attacks you get attacks due to combat reflexes and a high dex. Thus, I would say that the hydra would get 1 attack per head but could only take one attack on any individual target.
I am at work so I can't quote any sources and I don't remember anything special written in the hydra that overcomes this.
But, you are never allowed to have more than 1 attack of opportunity against an opponent in 1 round no matter how many times he provokes and no matter how many attacks you get attacks due to combat reflexes and a high dex. Thus, I would say that the hydra would get 1 attack per head but could only take one attack on any individual target.
I am at work so I can't quote any sources and I don't remember anything special written in the hydra that overcomes this.