Judges in the Supreme Court

The full complement of judges in the UK Supreme Court is twelve; but at the time of the Brexit case, Lord Toulson had retired and was still to be replaced. Hence only eleven judges sat on the case.

The question did not make this clear, or that it was specifically the number of judges that sat in the Brexit case that was required. IMHO, "twelve" should have been accepted as an answer (or the question should have been reworded).