I was always rather curious after putting up "The Top 10 Singles in Oricon History" about how the company that has been compiling all of those statistics and rankings in music treated songwriters. Well, I was able to track down the Top 5 Japanese composers of all time so far and will eventually come up with the lists for lyricists and arrangers. The J-Wiki article for Kyohei Tsutsumi provided the information which had originally been published at "Buzzfeed News" in January 2018.
As for the numbers, the Wikipedia article for Tsutsumi stated in its first paragraph that he was responsible for "...selling over 76 million units on the country's single chart from 1968 onwards.". I'm not certain whether those singles include the B-sides that he (and the rest of those composers) may have also composed. Plus from that quote, it's possible that non-single tracks from albums may have been excluded, so the list may really not be a wholly complete one. It's also been more than 2 1/2 years since it has been compiled so there's a chance that there may have been some shifting.
1. Kyohei Tsutsumi（筒美京平） 75.6 million units
2. Tetsuya Komuro（小室哲哉） 71.8 million
3. Tetsuro Oda（織田哲郎） 41.8 million
4. Keisuke Kuwata（桑田佳祐） 38.9 million
5. Tak Matsumoto （松本孝弘） 38.1 million
Furthermore, the J-Wiki list was generous enough to also include each composer's most successful single. Unfortunately, I have yet to write up an article for Matsumoto's (B'z) song but I will try to rectify that in the coming days (and I did).
Kyohei Tsutsumi -- (1979) Miserarete （魅せられて）
Tetsuya Komuro -- (1997) Can You Celebrate?
Tetsuro Oda -- (1992) Sekaijuu no Dare yori Kitto （世界中の誰よりきっと）
Takahiro Matsumoto -- (1993) Ai no Mama ni Wagamama ni Boku wa Kimi dake o Kizutsukenai（愛のままにわがままに 僕は君だけを傷つけない）