In considering the Ranger class, a fundamental question comes to mind that I wanted to run past the forum. Let's say a 5th level ranger has an Extra Attack and the Horde Breaker skill. Is he able to make two attacks using the Extra Attack ability, and one additional attack using the Horde Breaker skill if two enemies are within 5 feet of one another -- for a total of 3 attacks during his turn? Thank you in advance.
You have one regular attack and one extra attack. You can attack one target twice and one target once, or three different targets once. This is not quite the same as three attacks, as three attacks would allow you to attack one target thrice, while Horde Breaker requires that you attack a different target.
But yes, the total of attacks is 3.
ExtraAttack PHB p.92:
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
If you take the attack option on your turn you attack twice, either against the same target or against two targets.
Horde Breaker PHB p.93:
Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.
Any weapon attack you make during your turn (i.e. not as a reaction and not a spell attack) allows you to trigger Horde Breaker and attack a different target to the original using the same weapon as the triggering attack and within the range limits.
You get to choose which attack during your turn triggers the Horde Breaker ability. Sometimes a character gets an attack as a bonus action, for instance when it is wielding two weapons, and you could choose to trigger it from that attack for some reason.
So yes, a 5th level ranger with Horde Breaker can perform up to three attacks if they have two targets within the range limits.
In the case of a 5th ranger wielding two weapons, they could potentially get two attacks from the attack option with their on-hand weapon, an attack with the off-hand weapon as a bonus action and then a Horde Breaker attack triggered by any of those three for a total of four attacks.