Add a number in random rows in Excel

I have a Excel that have 780 rows with data. I like to add in one column (D) a number but I like to add it random rows and only 15 of them so if the row already have a number is going to find another one that does not have any.

It is for a game that I going to do and the number I going to add is week numbers (1-52) so each week I going to have 15 question.