I don't know many but I think Tsugumi Ohba would qualify. He has a few manga and 2 animes. I don't really know anyone else... Also Hiromu Arakawa has quite a bit.
Konosuba author had: -Konosuba -kemono michi as anime this season though it was original manga to anime -Sentouin from novel to manga.
Well in that case the guy who made Douluo Dalu cause he has so many titles and manga/anime/dramas under him, Tang Jia San Shao, unless language matters too.
Maybe 'Hosoda Mamoru' and 'Nisio Isin' (separate authors), I like some of their works (they got many) like monogatari series and Wolf Children
Tang Jia San Shao of Douluo series and several other novels/manhuas. Not sure if his work has been made into anime but with its popularity it's probably only a matter of time.
TJSS would beat any other author. Manhua = 12 zodiac protector god Child of light DD 1 DD 2 DD 2.5 DD 3 DD 4 HJC Jiu Shen Mad God Magic chef of ice and fire Only I shall be immortal Skyfire avenue Space speed star marks SYWZ The kind death god Zither Emperor Donghua = DD 1 Technically DD 2/3/4 have animated comic, but for fairness sake lets not count that
nisio isin: katanagatari (anime only), the forgetful detective series (alternative title: kitegami kyoko no biboroku), zaregoto (novel) has manga spin-off: zerozaki's soushiki's humanity test narita ryogho: baccano, fate strange/fake . he has a lot of other original novels and original manga though hasekura isuna: spice and wolf and sequel, magdala nemure
Nishio and Narita are definitely some of the most successful ones. Personally I prefer their works compared to index series. Monogatari series itself already have several adaptation, and each arc could be counted as a standalone series. Narita have Baccano & Durarara. His narative style aren't really suitable for anime adaptation tho. As for Hasoda Mamoru, I suggest you to look it up on google. He's one of the most renowed director alongside Makoto Shinkai.
edit: my mistake, yuu kamiya is the illustrator, not author (he illustrated these different series: a dark rabbit has seven lives, clockwork planet, no game no life) Keiichi Sigsawa: kino's journey, allison and trieze, Sword Art Online Alternative Gun Gale Online I found this guy randomly (don't really know him myself): Hideyuki Kikuchi: vampire hunter d, demon city Akatsuki Natsume: konosuba and spin-offs, sentouin hankenshimasu mizuki nomura: when hikaru was on earth, book girl