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?

The hydra can take them all at the same time in response to a single trigger or pace them out over a round in response to multiple triggers.

Here's the text of the feature we're talking about (from the hydra's stat block):

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

And here's an explanation of how reactions are used, when you get them, and how opportunity attacks are triggered (from the Combat chapter of the Basic Rules):

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's. The opportunity attack... is the most common type of reaction....When you take a reaction, you can't take another one until the start of your next turn.... You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

Combining these together, the hydra, unlike most creatures, has more than one reaction they can take per round (the extras can only be used for opportunity attacks, not Ready actions or other reaction options), regaining any spent reactions at the start of its next turn. There's no restriction to when those reactions can occur within that time frame, so they could happen simultaneously or separately over the course of the round, as the hydra chooses.

In addition, there is no rule that a provocation, such as moving out of a hydra's reach, can only incur one opportunity attack. This means that whether multiple creatures are taking opportunity attacks or one creature's multiple heads are doing it, they can still all happen at once in response to the same provoking trigger.

Yes

The Hydra contains the following in its stat block:

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

If Fighter A is in fact leaving the range(10') of each head, then each head is able to take its reaction for an Opportunity Attack.

Each head can take the reaction, but it gets to choose when it wants to. It can unload on the fighter, or it can let it go and save it for later in the round.

Spent Reactions

Do also note that once a head has taken its Opportunity Attack reaction, it has used a reaction and does not get another until the start of its turn (allowing other creatures to go in and out of range.) You will need to track the number of heads the Hydra has and how many reactions for Opportunity Attacks it has.

When you take a reaction, you can't take another one until the start of your next turn.

Yes.

The Hydra's heads function as separate entities in this case. You can assign attacks as desired, one or more attacks per opportunity. Just don't lose track of the opportunity attacks.

No

Lets look at the opportunity attack rules:

Opportunity Attacks

[...]

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

[...]

Suppose we remove the reaction cost.

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, [SNIP] make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

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.