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.