piano chord
C♯11
C♯1E♯3G♯5B♭7D♯9F♯11
Six notes in theory; in practice the third is usually dropped, because the eleventh sits a half step above it.
Written the other way round it is D♭11 — the same twelve keys on a piano, a different piece of notation.
What is true of it
C♯11 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.