piano chord
B♭11
B♭1D3F5A♭♭7C9E♭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 A♯11 — the same twelve keys on a piano, a different piece of notation.
What is true of it
B♭11 is B♭, D, F, A♭, C and E♭ — 1, 3, 5, b7, 9 and 11 counted from B♭.
On an instrument that cannot reach all 6, the 3, 5 and 1 go first, in that order.