It depends on how you look at it. On one hand, some purists might dislike the fan - fiction - like elements as it doesn't seem as polished or in - line with the original 'Avatar' series. However, from a different perspective, it can be seen as a way to expand the universe in new and unexpected directions. It gives the creators the chance to take risks and try out different ideas that might not have been possible in a more traditional approach. So, it's not an absolute bad thing.
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.