Confused about AoO's on mages

[Beretta]

So how many times a round do you allow it to use all its heads on an AoO?

If someone stands up from prone (AoO) and runs away (AoO), do they get bitten eight times... twice?

-Hyp.

When you put it like that...

Not so confusing when given some context. Thanks!

It would be almost worth it though just to see the look on the player's faces if that happened!

Or maybe not...

 

[MarauderX]

From 3.0 MM
Feats: A hydra’s Combat Reflexes feat allows it to use all its heads for attacks of opportunity each round.

Right on. That's one rule I will be rolling back to 3.0, at least in the case or some similar.

 

[billd91]

But that's the only exception given.

As far as I can tell, if someone makes a full attack and shoots four arrows at you, he provokes four AoOs.

-Hyp.

That's essentially right. It's because the attack itself provokes the AoO and not spending an attack action that provokes the AoO. By comparison, it's the move action, as long as it encompasses leaving a threatened square, that provokes the AoO.
I don't think I would call movement the only exception given. There are some things that trigger AoO that are actions that can be performed multiple times in combat (ranged attacks, unarmed attacks, disarm attempts, sunder a weapon) as a single full-round action. Each time within that full-round action, the act triggers an AoO. There are other things that trigger a single AoO when done as an action type (move, run, cast touch spell on 6 friends), either move, standard, or full-action.
I think if you view the table on p141 of the 3.5 PHB as the listing of events where each, if marked yes, is an example of a single triggering opportunity, then it's pretty easy to keep straight.