Some more, besides those mentioned in
www.talkclassical.com

How many composers actually have complete (or 95-99%)...
I know it's impossible to have every scrap or note of everything ever written, but I've come across a few box sets that are, for the most part, complete. Then I've come across others... Masterworks, Edition, etc. And when I look at reviews, they are far from complete. I've come across these...
- Alma Mahler - to my knowledge, only about 14 lieder are known. All the lieder can be contained on a single CD
- Abel Decaux - only wrote a very few piano works, including the published Clairs de Lune. Everything can be contained on one CD
- Boris Pasternak. His entire oeuvre - songs, piano works, a choral work - can be contained on 1-2 CDs, but I'm not aware of such a project.
- Julius Reubke's complete works (only piano and organ music has been preserved) can be bought, on 1 CD, played by Schöch.
- Julian Scriabin's could be on less than 1 CD too.