How many times can the effect of Hail of Thorns be activated?

Question

In a previous question I asked how many times a ranger can attack if he has horde breaker and extra attack. Now, if he is using a bow and the Hail of Thorns spell is active, can it be activated once for each attack? The description says:

The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition. In addition to the normal effect of the attack, the target of the attack and each creature within 5 feet of it must make a Dexterity saving throw. A creature takes 1d10 piercing damage on a failed save, or half as much damage on a successful one.

I think that this effect can be used once for each attack (so, 3 times maximum), but I am not sure about this.

Answer 1

You trigger the effect of this spell once.

The next time you hit a creature with a ranged weapon attack before the spell ends, this spell creates a rain of thorns that sprouts from your ranged weapon or ammunition.

Just as the spell text says, this will trigger the next time you hit a creature with a weapon attack. Not every time you hit a creature with a weapon attack. You cast the spell, then the next weapon attack you hit triggers the effect of the spell, and that's it.

Answer 2

Hail of Thorns can only be activated once

There's a clear distinction in the PHB between spells and features that work once versus spells that work multiple times. Spells that work multiple times will say something akin to "Each time you hit a creature with a melee weapon attack before this spell ends", whereas single use features and spells will say something like "The first time" or "The next time you hit a creature...". Hail of Thorns does say, according to (PHB p. 249):

The next time you hit a creature with a ranged weapon attack before the spell ends

enforcing that it is only usable once per cast.

You can, however, use Hail of Thorns twice in a turn.

To do so, you must cast Hail of Thorns as a bonus action at the end of a turn. On your subsequent turn, hit with a ranged weapon attack, activating your (first) Hail of Thorns.

Then take your bonus action to cast Hail of Thorns again before hitting with a second ranged weapon attack, activating your second Hail of Thorns of the turn. You can take a bonus action between attacks as described here.

Answer 3

Very simply the spell (PHB p. 249) states:

The next time you hit a creature with a ranged weapon attack before the spell ends

So the effect happens on the next hit and only the next hit by the caster using a ranged weapon, before the spell ends (Concentration, up to 1 minute).

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As to the spell duration, it is about timing. For instance the ranger could cast the spell out of sight and hearing, then sneak into range/line of sight of the target and shoot an arrow when ready without giving their position away from having to cast the spell. Not having a V component is a major thing, particularly if it is not an illusion, as is being a bonus action spell which would be other ways of doing it.