There are no clear signs indicating that 'the legend of korra book 5 graphic novel' will be released in the near future. The creators might be working on it in secret, or they might not have plans for it at all yet. Without any official news or hints from the production side, it's very difficult to predict when or if it will be released.
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.
Well, I'm not sure exactly as there's no official Book 5 yet. But if it were to exist, perhaps more adventures of Korra, new bending techniques, and deeper exploration of the Avatar world.
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.