piano chord
A♯11
A♯1C𝄪3E♯5G♯♭7B♯9D♯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 B♭11 — the same twelve keys on a piano, a different piece of notation.
What is true of it
A♯11 is A♯, C𝄪, E♯, G♯, B♯ and D♯ — 1, 3, 5, b7, 9 and 11 counted from A♯.
C𝄪 is a double sharp, and it has to be: every letter from A♯ upwards is used once, so the 3 cannot be written any other way.
On an instrument that cannot reach all 6, the 3, 5 and 1 go first, in that order.