piano chord
C11
C1E3G5B♭♭7D9F11
Six notes in theory; in practice the third is usually dropped, because the eleventh sits a half step above it.
What is true of it
C11 is C, E, G, B♭, D and F — 1, 3, 5, b7, 9 and 11 counted from C.
On an instrument that cannot reach all 6, the 3, 5 and 1 go first, in that order.