Well, there are fanfictions where the OC has a unique backstory. For example, an OC who was once a part of a circus troupe and has acrobatic skills. When she meets Mako, he's initially skeptical of her but as they get involved in the Avatar's adventures, he comes to admire her tenacity. Some fanfictions focus on the romantic aspect right from the start, with Mako being instantly attracted to the OC's kind heart and beauty.
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.