The plot should be engaging. It could involve a new threat to their world that only they can solve together, like a powerful dark spirit that is immune to normal bending attacks. Or it could be a political intrigue where their skills are needed to restore balance in their respective nations.
One popular 'azula korra fanfic' is 'The New Beginnings'. It explores an alternate universe where Azula and Korra have to team up to face a common enemy. The story is really engaging as it shows how their different personalities clash and then gradually find a way to work together.
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.