The pool of people hired to compose music is fairly small, akin to directors and series composers. While they often will compose the pop songs for OP and ED sequences, that’s typically separate project from the regular soundtrack. For most anime, the music will fill one or two albums. Yukari Hashimoto’s eight volumes of music for Mawaru Penguindrum (in addition to rearranging ARB’s songs for the in-universe idol group Triple H), is not the norm. Many composers are part of music groups, such as Maiko Iiuchi of I’ve Sound. It’s also not uncommon to have multiple composers work on a single series. Asami Tachibana composed songs for Haikyu!! and Gundam Build Fighters Try along with Yuuki Hayashi. While second seasons will use the same themes from the first with some additions, hey a counted as their own independent entry in this data.
The percentage of women composing music for Otaku TV series has had significant variance over the five year period from 2011 to 2016, though with the increasing number of series, the proportion trends towards a decrease. The debuts in this time period include Ruka Kawada (2013, Senran Kagura), kotringo (2015, Koufuku Graffiti), Asami Tachibana (2012, Robotics;Notes), Minako Seki (2012, Kingdom), and Yui Isshiki (2011, Dog Days).
Among the most active composers are Yukari Hashimoto (Mawaru Penguindrum, Gekkan Shoujo Nozaki-kun, Sore ga Seiyuu!) and I’ve Sound member Maiko Iiuchi (Wixoss franchise, Rewrite, Heavy Object). The next two are both composers who debuted in anime during this time period, Asami Tachibana and Ruka Kawada. The next for are generally well regarded composers have have been working on music composition for anime for a decent while longer.