How many ounces of yarn will I need to crochet an afghan from worsted weight yarn, 86"Lx 72"W?

It really depends on the type of yarn, the pattern, and how tight or loose you crochet. Personally, I would figure on about 84 ounces, which is 21... 4 ounce skeins. You can always return what you don't use, or keep it for another project. Maybe you could make a matching pillow with any left overs.