Though I don't know if this warrants a "warning," as new moongirl requested, I will label this as Zutara. It's my main ship, so ultimately, everything I write is Zutara. And yes, snarkhunter, laughing at the end is good ;)
The first assassination attempt happened before Zuko was even conscious. They commandeered a nobleman's house because Katara didn't think Zuko could take being transported to the palace so soon after being seriously injured in the final battle. She opened the door to his room to find a guard holding a pillow over his face. His heart exploded in his chest.
The second one was also while Zuko was unconscious. Toph was sitting with him when a nurse thought she could poison him. She was never the same again. The only reason she wasn't crushed to death was because Iroh wanted her to say who'd paid her. They didn't tell Zuko about these two until years later.
Someone tried to kill him during his coronation. Suki took care of that one. Fans are sharp indeed. No one fitted the man for a prosthetic arm. The assassination count? 45. In three months.
They tried ganging up on him, but eight months into his reign, Zuko had most of his strength back. One got hit with lightning. One was on fire. One got a dagger in his throat. He was up to 200.
The assassins weren't all Fire Nation. An Earth Kingdom man tried to stab him during a meeting, but was electrocuted when Zuko grabbed the arm with the dagger. This was only a week after he'd been ganged up on in the market, and this was now the 203rd attempt. No matter how hard they tried, the servants couldn't make the smell go away for months, and insisted Zuko use a different meeting room. Zuko continued to use that meeting room for the intimidation factor.
And these were only the ones that actually made it to Zuko. Countless ploys were taken down in parlor rooms and seedy dockside hotels before they could ever hand over the money to the assassin. These were not counted.
Sometimes they attacked during the night. Zuko let Mai deal with those. Better to have her aim deadly pointy things at people other than him. Numbers 290, 291, 292, and 293.
Once, someone opened the secret passage in his bedroom without using the secret knock the gang had agreed upon. Zuko and Katara attacked at the same time. They wouldn't risk Katara being discovered in Zuko's bed when she wasn't supposed to be. Zuko said he was alone when it happened because no one would be able to say there had actually been ice daggers present. That was 300.
The closest Zuko came to actually dying was when he was travelling through the Earth Kingdom countryside on his way to Ba Sing Se for the annual Peace Summit. There were ten attackers, but it was clear that the big guy with the hammer was to smash the Fire Lord to as many small pieces as possible. The hammer caught Zuko in the side, shattering two ribs and fracturing two more. Zuko put on a brave face during the opening ceremony. He, Katara, and Sokka were talking alone in the garden, and she playfully elbowed him in the ribs.. Toph actually cried when she was told how bad it was, and Katara had yelled at him until she'd lost her voice. Ozai laughed in his face and freely admitted to that one. He knew Zuko wouldn't have him executed because the Avatar decreed that he should remain alive and repent for his wrongs.
He was poisoned once, during the year of his and Katara's engagement, six years after becoming Fire Lord. They knew about it, had a plan in place, shut the palace down. This was orchestrated by the nobility, with Ozai's help. Even though Zuko knew he would be in no real danger, he still hesitated as he looked into the poisoned cup. He closed his eyes and didn't look at Katara as he drank it. It paid off. Seven nobles were implicated in the crime, four servants tried four separate times to kill him while he was sick in bed, and one guard thought better of it when she saw Ursa sitting on the bed, rubbing Zuko's back while a stable hand and an architect lay dead on the floor. He'd beaten the record of 414 previously held by Sozin, but Chief of Staff, Minister Hau kept count. Numbers 610-617.
Everyone captured alive talked. They talked a lot.
For a while, the gang played Guess Why They Want Zuko Dead in their letters and whenever they got together. Usurping the throne. Ozai still hates Zuko. Zuko's trying to ruin the Fire Nation. Zuko has ruined the Fire Nation. Ozai should be tried as a war criminal. Ozai still hates Zuko. Ozai wants to keep Zuko on his toes. Ozai's having a bad day. That bloodline needs to end. Zuko's not fulfilling his duty by getting married. Zuko has the audacity to marry a waterbender. That woman's a witch, get her away from him. If he's dead we can make her marry someone we want and still keep support from the South.
By the time their first son, Kurzu, was born, Zuko had an unprecedented 945 assassination attempts in seven years. They'd begun to trickle off after Zuko and Katara's wedding where they'd shown off just how powerful fire and water could be together, but occasionally someone would still have the urge.
Besides, there was a decreasing supply of would-be assassins willing to go up against him.