How many sheep does it take to produce a wool sweater for an adult human being?

None. Sheep are not used in the making of sweaters.

The fleece of sheep is used to make sweaters, following scouring, carding, spinning and dyeing.

As a rough rule of thumb, it takes about 600g of worsted weight yarn to make an adult sweater. As the process of making yarn is lossy – up to 50% loss by weight – this means roughly 1.2 kg of fleece is used to make a sweater. Different varieties of sheep produce different quantities of fleece, usually between 1 and 4 kg, so a worsted-weight sweater uses the fleece of between 0.3 and 1.2 sheep.

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