It really depends on what size you want your quilt to be. I like to use square quilts on my queen-size bed and put pillows on top. The mattress is 60" X 80", so a quilt top that is 95" X 95" will hand down about 17" on the sides and foot of the bed.
In order to make that quilt ALL 5" blocks with no sashing or borders, you would need 361 blocks (19 rows of 19 blocks) . Personally, I would make it 20 X 20 blocks, to allow for shrinkage in the quilting process.
If you add borders, you can use fewer blocks.