How many yards of yarn does it take to crochet a twin size afghan using an H hook and single crochet stitches?

You would need to know what yarn you are using - sport, worsted, bulky - and what pattern. Different stitches will use different amounts of yarn. Single crochet is more dense than double crochet and would use more yarn to make the same size finished product. Afghan stitch is also dense with same result.

With worsted yarn I would estimate 2500-3000 yards. That is a really rough estimate, however, without more information. Your chosen pattern should specify the yarn to be used and how much. If you want to substitute yarn, you can compare other yarns with the same weight, number of ounces and yards.