A normal creature only has one reaction that may be used as a single attack of opportunity. A Hydra, though, has one of these reactions per head.
My question would be: may several reactions be combined in reaction to the same action and thus allowing several attacks of opportunity at once?
Scenario: Fighter A attacks Hydra with five heads and then tries to leave, thus provoking an opportunity attack. Now, as a reaction to this, may the Hydra attack Fighter A only once or up to five times?
Best Answer
No
Let's look at the opportunity attack rules:
Suppose we remove the reaction cost.
You'll note that this is very clear that you don't get to make 1 million opportunity attacks when someone leaves your reach.
You can make an opportunity attack when the trigger occurs. It doesn't say "you can make as many as you want".
Adding a cost -- an extra restriction -- doesn't suddenly make that paragraph let you make infinite opportunity attacks by paying the cost repeatedly. An extra restriction doesn't remove other requirements.
So a Hydra can make one opportunity attack when provoked by someone leaving its reach. It costs one reaction. Unlike most creatures, it has multiple reactions, so can do this again later.
Weak support from a tweet:
This is weak, because you could read this as "one attack per opportunity attack, and you do 10 opportunity attacks to one opportunity attack trigger; see, no problem!", but that would be an extremely strange way for Crawford to say it.