WARNING: The following contains minor spoilers for the My Hero Academia manga by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
As a general rule of thumb, it's usually one Quirk per person in My Hero Academia. However, some extraordinary circumstances have resulted in a few characters welding more than one. Here's a list of the select few heroes who have used -- or currently use -- multiple Quirks.
Katsuki Bakugo: Explosion and (Briefly) One For All
Bakugo's Explosion is one of the stronger Quirks in My Hero Academia, but Heroes Rising proved it wasn't all-powerful. When he and Deku faced Nine, the only way they could defeat the villain was by sharing One For All. Bakugo quickly adapted to its use though, and together they made short work of Nine. Additionally, doing this amplified Bakugo's own Quirk and made his explosions even more powerful. While he held it, his blasts were on a whole other level.
Gigantomachia Uses an Incredible Seven Quirks
As All For One's most loyal servant, it's only natural that Gigantomachia would receive a few power-ups, and his master spared no expense in making him a beast. His naturally tough constitution allows him to endure seven Quirks, a number that would normally be impossible for one person to withstand without serious body modifications. Gigantomachia's combination of Quirks makes him almost impossible to defeat and earned him the nickname "walking natural disaster," as he leaves only destruction in his wake wherever he goes.
Neito Monoma, Class 1-B's Ultimate Copycat
Class 1-B's Phantom Thief has one of the most enviable Quirks in My Hero Academia. His Copy allows him to replicate the Quirks of up to four different people he touches for 10 minutes. During that time frame, he can use all the Quirks he's copied to his heart's delight. This freedom allows him to combine them for super moves that would be otherwise impossible to perform. When combined with his immense intellect and knack for figuring Quirks out, Copy makes Monoma quite a tricky customer to deal with in battle.
Kurogiri, Shigaraki's Mysterious Guardian
Tomura Shigaraki's guardian appears to have only one Quirk -- the sinister-looking Warp Gate. However, Season 5 of My Hero Academia revealed Kurogiri's true identity as a Nomu. Furthermore, his Warp Gate combines several different Quirk factors that allow him to create his shadowy portals.
Kurogiri, formerly a UA student and best pal of Eraser Head and Present Mic, used to be known as Oboro Shirakumo. He was a hero-in-training and his Quirk, Cloud, forms the base of Warp Gate. The exact number of Quirks combined with Cloud to create it hasn't been revealed yet but given Warp Gate's versatility and accuracy, it shouldn't be less than three.
Izuku Midoriya Discovers One For All's Origins
When Deku first received One For All near the start of My Hero Academia, he had no idea just how much danger and responsibility would come with the Quirk. Apart from putting him in the crosshairs of Japan's most dangerous villains, One For All had the Quirks of all its previous users locked within it and he now has to master them all.
He's only used Blackwhip so far in the anime, but the other Quirks shouldn't be too far behind. In the manga however, Deku has unlocked several more: Fa Jin, Smoke Screen, Float and Danger Sense. These Quirks make him one of the most powerful people currently in My Hero Academia.
Tomura Shigaraki Goes Plus Ultra With Multiple Quirks
As part of the process that was supposed to transform him into All For One's next vessel, Tomura Shigaraki was fitted with a large number of Quirks that had been previously stockpiled. He was awoken prematurely yet even in his incomplete state, Shigaraki was more than a match for several top heroes teaming up against him. From EMP blasts to regenerative powers, the new Plus Ultra Shigaraki's Quirks are diverse and powerful.
Shoto Todoroki, Born of Fire and Ice
Shoto is the singular instance of someone being born with multiple Quirks in My Hero Academia. Such a rare occurrence didn't happen by chance, of course. Endeavor wanted a child who'd inherit his flame Quirk but without his tendency to overheat from using it. His solution was to procreate with Rei, who had an Ice Quirk, and together they had a son. Shoto's Fire and Ice are distinct enough to be classified as two different Quirks; they're vastly different powers and he can use either one independently of the other.
Lady Nagant, "Gifted" a Second Quirk From All For One
All For One tasked the rifle-armed assassin Lady Nagant with bringing Deku to him. To make her work easier, he granted her his own Air Walk Quirk, allowing her the power of flight and all the advantages it came with. Unfortunately, this gift came with a price -- All For One had rigged it with a trap set to activate should her convictions ever waver.
Nine, Who Used All For One in Heroes Rising
The main antagonist of My Hero Academia: Heroes Rising, Nine had the capacity to wield eight additional Quirks in addition to his Weather Manipulation. Before he could steal enough fill every slot however, he was defeated by Bakugo and Deku using Two One For All. Nine gained his synthetic All For One Quirk from Dr. Garaki, who used him as a test subject to determine whether the process to grant Shigaraki his mentor’s Quirks would succeed. The operation was a success, but Nine was unable to use his Quirks to their fullest extent because of his deteriorating health.
The Previous Users of One For All
Unlike Deku and All Might, the previous users of One For All had their own Quirks in addition to the power-stocking ability. Since OFA wasn't as powerful in their time as it is now, they used it in tandem with their original Quirks, strengthening it in order to pass it down until it was strong enough to finally defeat All For One.
All For One Has Used At Least 10 Quirks
The villain boss is singlehandedly responsible for all but two of the entries on this list, whether directly or indirectly. All For One's initial Quirk allows him to steal those of others and either use them himself, or transfer them to those willing to swear loyalty to him. His fascination with Quirks has led him to experiment on them extensively.
He created the Quirk monsters known as Nomu with Dr. Garaki, but that wasn't the extent of their collaboration. Together, they devised ways to create synthetic Quirks through experiments and shelve them for later use. The exact number of Quirks he's stolen over the years is unknown but in his battle against All Might, he used no fewer than 10. But given All For One's age and greed when it comes to Quirks in My Hero Academia, the number could be well into the hundreds.