The amount of yarn you need will depend on what size blanket you are making. Blankets of the smaller size range (toddler or smaller) may require less than 500-700 yards, while larger blankets can require anywhere from 1500-3000+ yards of yarn.
Some crochet stitches or stitch combinations like bobbles, puff stitches, basketweave patterns, etc. are “yarn eaters” meaning they use a lot of yarn.