One great korra x zuko fanfic is 'The Fire and the Water'. It beautifully weaves their different personalities and abilities together. The story takes place in a post - war world where they have to work together to rebuild. Another is 'A New Alliance' which focuses on their growing friendship that slowly turns into something more. It has really interesting dialogue that makes you feel like you're right there with them.
The contrast between their characters. Korra is brash and bold, while Zuko has a more complex and brooding personality. When these two are paired in a fanfic, it creates a lot of interesting dynamics. Their different bending abilities also add to the allure. Fire and water, which are opposing elements, can represent their relationship in many ways.
The Legend of Korra comics are definitely worth a read. They capture the essence of the franchise and bring fresh perspectives. The quality of the storytelling and the visuals make them a great addition to the series.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.
To be honest, I don't have an exact figure. But you can look up on popular comic book databases or ask in comic book enthusiast communities to get a clear idea of how many The Legend of Korra comics are out there.