Part of an ongoing series of 5e Rules notes.

So how many spells can you cast in a turn?

The answer? Everyone say it together: It depends!

But first, a minor digression (that isn’t)

Terminology:

Round:  A cycle round the table, ordered by Initiative, during which each PC/NPC takes a turn.

Turn: A PC/NPC’s spot, ordered by Initiative, when the PC/NPC can move and take actions. Each PC/NPC gets one turn per round.

So, each round, your character gets to take their turn. There are parts of the round that are not your turn (but during which you may react).

This is important in understanding the below.

Okay, let’s answer the question

Let’s start with the Sage Advice Compendium:

Is there a limit on the number of spells you can cast on your turn? There’s no rule that says you can cast only X number of spells on your turn, but there are some practical limits. The main limiting factor is your Action. Most spells require an Action to cast, and unless you use a feature like the Fighter’s Action Surge, you have only one action on your turn.

By default, you can, pragmatically, cast one normal spell per turn, using the Cast a Spell Action. Most spells have a casting time of 1 Action. Easy peasy, right?

But what about spells that you can cast as a Bonus Action? There aren’t many, but they say it right in the spell timing. (It’s worth noting “Action” and “Bonus Action” are not interchangeable; if something is one, it cannot be done as the other.) So if you cast a Bonus Action spell, can you then cast a regular Action spell?

Yyyyeah, but it creates some limitations:

Bonus Action
A spell cast with a Bonus Action is especially swift. You must use a Bonus Action on your turn to cast the spell, provided that you haven’t already taken a Bonus Action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 Action.

and

If you want to Cast a Spell that has a casting time of 1 Bonus Action, remember that you can’t cast any other spells before or after it on the same turn, except for cantrips with a casting time of 1 Action.

So, if you cast a spell, such as Healing Word, with a Bonus Action timing, you can cast another spell with your Action, but that other spell must be a cantrip, not a levelled spell.

But what about Sorcerers?

Sorcerers have a metamagic tool, though, called Quickened Spell. 

Quickened Spell
When you Cast a Spell that has a casting time of 1 Action, you can spend 2 Sorcery Points to change the casting time to 1 Bonus Action for this casting.

So that lets you cast a levelled spell of 1 Action casting as a Bonus Action instead. But that doesn’t get rid of the limitation above: “If you want to Cast a Spell that has a casting time of 1 Bonus Action, remember that you can’t cast any other Spells before or after it on the same turn, except for cantrips with a casting time of 1 action.”

Again, this is confirmed in the most recent Sage Advice Compendium:

Does Quickened Spell allow a sorcerer to cast two spells a round of 1st level or higher?
No, the sorcerer must follow the rule for casting a spell as a Bonus Action and casting another spell on the same turn; the other spell must be a cantrip with a casting time of 1 Action.

Along this line, there is an Epic Boon that allows a spell you know to be turned into a Bonus Action spell permanently. But we needn’t delve into that right now …

To sum up …

So, what are valid combos?

• YES: Cantrip @ 1 Action + Spell @ 1 Bonus Action
• YES: Cantrip @ 1 Action + Cantrip @ 1 Bonus Action
• NO: Spell @ 1 Action + Spell @ 1 Bonus Action
• NO: Spell @ 1 Action + Cantrip @ 1 Bonus Action

Unless noted otherwise, the Bonus Action can go before or after the Action, but remember that you can only cast something as a Bonus Action if that’s its timing in the spell book, or if you use Quickened Spell to cast it.

But what about spells that give Bonus Actions?

If a spell gives you a Bonus Action, using that Bonus Action does not trigger this limitation effect (because you aren’t casting the spell in the Bonus Action). For example.

The rule on casting a spell as a Bonus Action (see PH, 202) applies only on the turn you cast the spell. For example, Spiritual Weapon can be cast as a Bonus Action, and it lasts for 1 minute. On the turn you cast it, you can’t cast another spell before or after it, unless that spell is a cantrip with a casting time of 1 Action.

Until Spiritual Weapon ends, it gives you the option of controlling its spectral weapon as a Bonus Action. That Bonus Action does not involve casting a spell, despite the fact that it’s granted by a spell, so you can control the weapon and cast whatever spell you like on the same turn.

But what about Action Surge?

One further edge exception to this is if you are a spellcaster who’s taken a couple of levels of Fighter (or are doing the Eldritch Knight fighter subclass).

Action Surge
Starting at 2nd Level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one additional Action.

As noted by the Sage Advice column, using Action Surge would give you two Actions on that turn, and both of them could be Cast a Spell. And that wouldn’t be limited to cantrips:

If you cast a second spell using Action Surge, you aren’t limited to casting a cantrip with it.

If you also cast a Bonus Action spell of some sort, though, those regular Action spells would be limited to cantrips (both of them). Because, again,

If you want to Cast a Spell that has a casting time of 1 Bonus Action, remember that you can’t cast any other spells before or after it on the same turn, except for cantrips with a casting time of 1 Action.

Well, then, what about Reaction Spells?

The rules about spellcasting actions also get bumped about by Reaction spells. Some spells (such as Shield) can be cast as a Reaction. You only get one Reaction per round, and Reactions (to others’ actions) usually take place outside of your turn. But … not always.

Reaction Timing
Certain game features let you take a special action, called a Reaction, in response to some event. Making Opportunity Attacks and casting the Shield spell are two typical uses of Reactions. If you’re unsure when a Reaction occurs in relation to its trigger, here’s the rule: the Reaction happens after its trigger completes, unless the description of the Reaction explicitly says otherwise.

Once you take a Reaction, you can’t take another one until the start of your next turn.

and

Reactions
Some spells can be cast as Reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a Reaction, the spell description tells you exactly when you can do so.

There are some weird edge cases where you might end up using a Reaction spell on your own turn. E.g., on my turn …

1. I Cast a Spell Fireball at the orcs.
2. I take a step forward, coming into range of the enemy wizard.
3. The enemy wizard fires off his Readied action of “Cast a Spell Magic Missile If I Step Within Range.”
4. I React with a Shield spell.

I react on my own turn, and that’s fine. And there’s no problem with effectively my casting two spells (my Action and my Reaction) because there’s no limitation on that; none of the conditions discussed above come into play because this doesn’t involve a Bonus Action.

But consider this case:

1. I use a Bonus Action to cast a Shillelagh cantrip.
2. I Cast a Spell Flame Bolt cantrip at the orcs (since I have already cast as a Bonus Action, I can only do a cantrip as my Cast a Spell action).
3. I take a step forward, coming into range of that enemy wizard.
4. The enemy wizard fires off his Readied action of “Cast Magic Missile If I Step Within Range.”
5. I React with a Shield spell … but I can’t.

Because Shield is not a cantrip and because I cast a Bonus Action spell and I can’t cast another leveled spell on my turn once I’ve done that. Again, as the rules say, once you’ve cast a spell as your Bonus Action, “You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.” That includes Reactions on my turn, just like it includes the second Action in an Action Surge.

If the enemy wizard acted right after my turn and fired the Magic Missile, then the Shield could be cast as a Reaction just fine, because I would not be Reacting on the same turn as when I cast a Bonus Action spell. Zany, but true.

As a further note, I am not sure if there are any Reaction Cantrips. As of 2014, at least, Jeremy Crawford was able to say:

Casting a Bonus Action spell does preclude casting a Reaction spell on the same turn.

In short …

So the answer to “How many spells can I cast on a turn?” seems to be:

• Three levelled spells (Cast a Spell, Action Surge+Cast a Spell, Reaction) or
• Three spells (Cast a Spell cantrip, Action Surge+Cast a Spell cantrip, Bonus Action)

If you don’t want to get into Action Surge, it looks like the number is two.

Bonus Action spells are really powerful in the flexibility they provide (more than one spell in a turn!), but they also gum up what else you can do, spellcasting-wise. Caveat incantor.