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.
One great piece of Legend of Korra male SI fanfiction is 'The New Avatar in Republic City'. It follows the story of a male self - insert who arrives in the world of Legend of Korra and has to navigate the complex political and bending landscape. He teams up with some of the show's characters and tries to bring about positive change in the city.
Some great Legend of Korra adult fanfiction might involve the older versions of the characters facing new challenges. For instance, Korra having to deal with a new spiritual threat that requires her to use all her skills and wisdom she has gained over the years. There could also be fanfictions that center around the cultural exchanges between the different nations as the characters grow older and more involved in diplomatic matters.
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.