Just as one method of estimating, I went to the Coats and Clark web site to see what they had listed for their Red Heart yarns.
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www.coatsandclark.com]
In the "medium" category, Red Heart Super Saver 7-oz skeins (commonly found at Walmart) are listed at 364 yards, or approx 52 yards per ounce; Eco-Cotton Blend is listed at 145 yards for 3 ounces, or approx 48 yards per ounce.
For Red Heart, I figure an overall average of 50 yards per ounce for worsted weight, so 38 ounces would be approx 1900 yards.
Of course, depending on brand, "your mileage may vary" (YMMV). You may want to check some other yarn companies also. (For example, Bernat seems to average approx 55 yards per ounce; Lion Brand varies from approx 48 to 65 yards to ounce.)
If you're still not sure, buy an extra skein or two of your chosen yarn, to assure you have enough of the same dye lot.