Can you knit a scarf or sweater using more than one ball of wool?

Yes. Depending on the size of the sweater and the weight of yarn used, a knitted sweater may use up to 1000 or 2000 yards of yarn. The average ball of worsted weight yarn contains about 200 or less yards, so it is not only possible, but necessary to knit a sweater with more than one ball of yarn.