Seriously, this depends on the length and weight of the skein of yarn you choose. And the math has to be done, if you're not strictly following a pattern with the yarn specified.
Get some yarn you like and crochet a sample square, measuring how much yarn you use for the sample (you can crochet the sample, measure it, and then unravel it to measure the length).
You divide the blanket size by the sample square size, and multiply that result by the yarn length of your sample to find out how much yarn you need and how many skeins to buy.
Or, find a pattern you like in the right type/weight of yarn you like, and see how much the pattern calls for. You still have to do the math to figure out how much that translates into the skeins of yarn you're buying, if it's different from the yarn called for in the pattern.