This is a very cute quilt.
Now on to the how much thread question....
I do a lot of quilts in this size range and it typically takes me 1 -2 spools of King Tut (500 yards / #40/3), but I almost always use the same thread in the top and bottom. In addition, most of the quilting is free motion that is 1-2 inches a part.
In your case I would go with 2 spools to be on the safe side and to allow for practice and design changes.