It has been many years since I've made one of those. I am guessing maybe 15-20 at least, depending on the size of the skeins you choose.

If you have crocheted other granny squares in the same yarn, weigh a square on a kitchen scale. Now, measure the square on both sides to find the area.

A standard queen-sized blanket would be 90x90 or 1800 sqare inches. Divide that by the area of your square. Multiply the answer by the weight of your square. Divide that by the weight of one skein and you will know how many skeins to purchase. I suggest purchasing a couple of extra, just in case.