A mangaka (漫画家, マンガカ), a shorten term of manga artist (マンガ・アーティスト) is a Japanese term for cartoonist. Outside Japan the term is only specific to people who draws and/or writes for manga.
The number of manga artists in Japan is extremely large, with several thousand active manga artists alone, and it is said that there are tens of thousands of people who have published manga or had their work published in manga magazines, but no one knows the actual number.
There are a wide range of ways to become a professional manga artist. The most common way is to apply for awards sponsored by magazine editors or to submit your work to them, but you can also be scouted by people who are active in coterie magazine activities or who publish work on the web.