
What is Music for Eight? I can’t find it listed here
https://johncage.org/pp/John-Cage-Works.cfm
It's
Music for ...I have this recording, and also a
Music for Seventeen version on the old Mosko/SF Contemporary/Newport Classics release.
Quoting notes (by Alan Rich) from the latter,
"
Music for ... can be performed by an indeterminate number of players (17 in this case). There is no written-down score, only a set of parts for flute, clarinet, trombone, three percussion, piano, violin and cello...(discussion of time brackets)..."
Notes to the Ensemble Avantgarde recording say that it's OK to omit some of the parts, which is good since otherwise
Music for Eight would be impossible (9 parts listed above).

It's included at your link:
https://johncage.org/pp/John-Cage-Work-Detail.cfm?work_ID=133which lists 17 available parts

. Presumably revised since the date of the Mosko liner notes.