One possible plotline could be that they are forced into an unexpected situation where they have to deal with the pregnancy together despite their differences. Another could be that it's a result of some magical or mysterious event that binds them in this situation, and they must navigate their new relationship while also dealing with the reactions of the Avatar world.
In the official Avatar universe, Tarrlok and Korra have a rather adversarial relationship at times. So a fanfiction about them being pregnant is quite a departure. But it could be an interesting concept for those who like to dig deeper into character relationships. Some fans might use it to explore themes of redemption for Tarrlok or a different side of Korra's character. However, overall, it's not a mainstream idea within the fandom.
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.