Most recordings you own by a single artist, group, or composer

Nice, Frank.
Thank you.

Ludwig van Beethoven, followed by Amadeus Mozart and next I think Johannes Brahms. 
"Lieben Sie Brahms?" 🤔
As a German born early Baby Boomer, I guess not too surprising its not Guns & Roses, or hm Red Hot Chilli Peppers... 😉

Original 6-eye Columbia for "Time Out" and "Time Further Out" is obtainable because Columbia is a big label that released a lot of copies of their major stars (unlike hunting down original Blue Note).  I think they both sound really good and are better than the reissues (probably an issue of the master tapes degrading a bit over time).  Columbia did fantastic sounding recordings back then.  When I want to show someone how little has been done in sixty years to improve the state of the art in recording, I take out an original 6-eye Ellington "Blues in Orbit" (stereo), to make my case.

I accidentally stumbled onto the chalet way up on Red Mountain in Aspen CO where Dave Brubeck and his group occasionally practiced and hung out. They were not there at the time, only the caretaker who let us listen to the stereo system. Six or seven racing Ducati’s in the garage. The stereo system was all tube Macintosh electronics, a tall rack of it, not the usual stuff, either. Ablaze with blue. At least 4 huge Macintosh speakers the size of refrigerators and some kind of monster mid range horn, maybe JBL. Brubeck’s concert grand piano was hooked up to a digital time delay system that my brother, demonstrated. There was a bunch of Joshua Light show stuff too that we activated with some Hendrix and Get Yer Ya Yas Out, almost too long ago to remember, back in 1970.

Mahler.  At least 4 or 5 versions of each of the symphonies (some more), plus all the other stuff.