You cannot cast two cantrips during one action. That’s why many cantrips scale as you level. To further explain what you read, if you cast a spell as a bonus action, you can only use your action to cast a cantrip.
Can you cast cantrips unlimited times?
Cantrips are free magic — if you know a cantrip, you can cast it as many times as you want, whenever you want. Cantrips don’t need to be prepared, and they don’t use up Spell Slots.
Can you keep casting cantrips?
No. A Cantrip will (most likely) still have a casting time of an Action, and so they can only cast them when they have an action available, which is only on their turn. The “at will” part of the wording means it doesn’t cost them one of their spells for the day.
Can you cast Eldritch Blast twice?
Unless they’re also a Sorcerer with Quickened Spell, have Action Surge from levels in Fighter, and/or have that magic item… they’re not allowed to cast Eldritch Blast twice in the same turn.
Can you Metamagic cantrips?
Yes, you can use Quicken Spell to cast cantrips. As you’ve noted, Quicken Spell requires a spell with a casting time of one action. Cantrips are defined as: A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance.
Do Cantrips automatically hit?
No. Cantrips are spells, just like any other spell.
Do Cantrips count as actions?
No, casting a cantrip replaces all of your attacks for the turn. Each turn (by default) you have one action to use. A few examples of actions are: Attack, Cast a Spell, Dodge, and Dash (see the Actions in Combat section of the PHB for more).
Can Eldritch Blast be cast as a bonus action?
Because of the attack action already taken, you can cast eldritch blast as a bonus action ONLY if you have a way to make eldritch blast’s casting time a bonus action. It’s normal casting time is one action.
