Dear God. Is this what we have come to.
Asking how many singles it takes to get a number one at xmas.
Just take a step back and think about what you have just asked, how it looks to other people - and what you think the answer might be for an xmas number one - or any number one for that matter. Unless we have misinterpreted what you were asking, why would you think it would be anything other than at least one more than a No.2 place.
And unless you know what number 2 is (as a figure) - you will never know an exact figure for achieving a no.1