Gonna say for purposes of this thread, even if you can sing the soprano portion in your lovely, rich, baritone, that still doesn’t make you a soprano.
You don’t include voice parts he used to be able to sing when he was 12? 
Here’s a extract of an exquisite choral piece by Bach for double choir (8 parts) and organ:
Two short sections from a larger piece:
Komm, ich will mich dir ergeben (Come, I will give myself to you)
Du bist der rechte Weg, die Wahrheit und das Leben (You are the right path, the truth and life)
If you can manage to overlook the religious sentiments, it’s a beautiful piece. 
Todd’s “Hermit of Mink Hollow” is another album where he wrote, arranged, produced, and performed the entire thing by himself.
The Ever Popular Tortured Artist Effect is another.
ETA: I’m a big fan. 
So we can rule out Mozart singing the “Queen of the Night” aria, accompanied by piano quartet.
Lol, yes.
Came here to post those two points in that sequence. ::darn! snapping fingers emoji::
Known more as a performer than composer, Niccolo Paganini could just tear though stuff, like his 24 caprices.
Liszt wasn’t too much of a slouch either.
Yeah, but now we’re getting away from the “for multiple instruments” stipulation.
Indeed, noted - my two don’t fit the OP.
The OP appreciates all links Paganini.
AFAIK, Paganini didn’t play any instruments beside violin (and by extension, viola). He composed duets with guitar and with bassoon, but I haven’t seen evidence that he played either. At any rate, he was no Mozart.
Haven’t really explored his piano quartets, thank you for this - it’s been an enjoyable journey.
As far as 70’s rockers go, Roy Wood made some pretty complicated music, performed everything himself and played a vast array of instruments, including many from the classical scene: vocals, guitar, bass, drums and keyboards, but also cello, bassoon, flute, clarinet, oboe, French horn, mandolin, bouzouki, recorders, bagpipes and double bass.
I think Roy Wood is the champion in his own class, judging “most complicated” notwithstanding.