D&D 5E Casting more than one spell in a turn/round

[Caliban]

If you want to waste a lot of class levels - Sorcerer 3/Fighter 2/Warlock 2 allows you to cast Eldritch Blast 3 times in one turn.

Quickened from Sorc = bonus action, regular action, and with action surge.

If you hit them with Hex on the previous round, that is a lot of attacks at 1d10+1d6+charisma (due to Agonizing Blast). 6 attacks at lvl 7, 9 attacks at lvl 9, 12 attacks at lvl 17.

 

[Cognomen's Cassowary]

It is possible, albeit not likely nor always desirable, to cast three fireballs in a single turn using your action, action surge, and rolling a wild-magic surge.

 

[Lanefan]

It is possible, albeit not likely nor always desirable, to cast three fireballs in a single turn using your action, action surge, and rolling a wild-magic surge.

After which you find, to your dismay, that the target of all that has fire immunity...

That said, doesn't a wild magic surge replace the spell being cast with the surge effect? Thus, casting fireball and having it WMS into a fireball still only gives one fireball.

 

[Caliban]

After which you find, to your dismay, that the target of all that has fire immunity...

That said, doesn't a wild magic surge replace the spell being cast with the surge effect? Thus, casting fireball and having it WMS into a fireball still only gives one fireball.

Nope. It's an additional effect.

 

[Cognomen's Cassowary]

After which you find, to your dismay, that the target of all that has fire immunity...

That said, doesn't a wild magic surge replace the spell being cast with the surge effect? Thus, casting fireball and having it WMS into a fireball still only gives one fireball.

Nope, the check for a surge happens after the spellcast, and surge effects happen in addition to the triggering spell.

Come to think of it, you could cast a fourth spell in that same turn: absorb element as a reaction, to take the edge off fireballing yourself.

 

[Blue]

* Someone attacks you. As reaction, you cast Shield.

This is not happening on your turn, this is happening on their turn. No interaction with anything else.

A round is everybody's actions all together. A turn is when you go. That's why a rogue can get another sneak attack on an opportunity attack - SA can only be used once per turn, and it hadn't be used on that foe's turn yet.

* Your initiative comes up.
* You cast Healing Word as a bonus action. (Alternate version: you cast Magic Missile as a bonus action, using the Quickened metamagic.)
* Your main Action can be "Cast a Spell", but now that only can be used to cast a cantrip. You cast Shocking Grasp on your immediately-adjacent foe.
* You move away from your foe, up to your base Speed. Your foe cannot take an Attack of Opportunity, because the Shocking Grasp prevents them from taking Reaction.

All this is good.

* You then take an Action Surge. Your extra Action is "Cast a Spell", and on THAT action, you can cast any spell. Now that you're out of your foe's melee reach, how about Scorching Ray? (Variant: Empowered!)

Nope. It is still during your turn, regardless that you have mulple actions on that turn. Since you cast a Bonus Action spell on your turn, your Action (or Actions in this case) must be cantrips.

 

[Blue]

I still question the

Bonus Action
"you can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action but the Sorcerer can cast a Spell with the casting time of 1 Action if the Sorcerer uses the Metamagic Option of Quickened Spell which cost 2 Sorcery Points as a Bonus Action"

Remember the Metamagic twists the spells to suit your needs

The Quickened Spell says nothing about the Spell has to be a Cantrip

It doesn't need to, that's under the general rules of casting. PHB page 202 there's a whole stanza on Bonus Action casting.