In these fanfictions, Naruto characters might become more cautious in a harem situation. For example, Naruto himself, who is usually brash, might think twice before acting as he has to consider the feelings of multiple love interests. In Legend of Korra, Korra could become more diplomatic. Instead of her straightforward, head - on approach, she might have to use more tact when dealing with the different personalities in the harem.
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.