One of the popular ones is probably 'That Time I Got Reincarnated as a Slime'. It has an interesting premise where the main character is reincarnated as a slime in a fantasy world. The story is filled with unique characters, like Rimuru, and explores concepts such as nation - building and different forms of magic. It has a great blend of action, adventure, and humor, which attracts a wide range of readers.
The number of Tensura light novels in Japanese is not fixed. It depends on the author's output and the publisher's schedule. You might want to keep an eye on the latest news from reliable sources for the most accurate count.
The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.