I thought OP might be referring to the Club ID, which is specific to each person who is an owner no matter how many contracts that individual might own. However, the Club ID is only 6 digits with a decimal point followed by a single digit.
The screenshot looks like it’s asking for the 12 digit number to be entered twice, and it seems that
DVC uses the terms Membership number and Member number interchangeably for that 12 digit number.
At least I hope not - tomorrow will be my first time trying for MM tickets under this new system, so I’ll have both my Club ID and my Membership Number handy!