Ok so I am not exactly sure if you want to know how many whole cups can be added or simply how many 2/3 cups make up 10, but heres what I found.
So to find how many 2/3 is in 10 cups you simply have to divide
10/(2/3) = 15
So 10 cups of flour is 15 (2/3) cups of flour.
Hope this helps :)