结合Combat Reflexes的说明，我觉得上面那句话就没什么可歧义的了。
COMBAT REFLEXES [GENERAL]
Benefit: You may make a number of additional attacks of opportunity equal to your Dexterity bonus.
With this feat, you may also make attacks of opportunity while flat-footed.
Normal: A character without this feat can make only one attack of opportunity per round and can’t make attacks of opportunity while flat-footed.
Special: The Combat Reflexes feat does not allow a rogue to use her opportunist ability more than once per round.
战斗反射的
主要意义就是每轮可以多次做出AO攻击。但要注意到，不管几个头，海德拉的Dex都是12。也就是按照正常的战斗反射规则，它每轮还是只能做一次AO，那么除了措手不及时也可以AO外，就没什么其它意义了。
因此，结合海德拉的专长、属性和这个特别说明，飞了提到的那句话就只能是指海德拉的战斗反射是使它每轮都可以用每个头做一次AO，也就是AO攻击的次数等于其头的数量。除此以外，还有什么足够合理的解释么？
PS：
Feats: A hydra’s Combat Reflexes feat allows it to use all its heads for attacks of opportunity.
注意这里用的是复数，表明做出的AO不止一次（如果按海德拉自身的Dex，那么是只有1次的），也就是一次一个头，最多AO12次。如果这里是说AO一下12个头咬过去，那么就应该用“to use all its heads for
an attack of opportunity”了，对吧？
