I wouldn't say "rear". Most farmers would say "raise". Saying, "When he was a farmer, he raised a lot of sheep" would probably be the more common expression. It would also be common in everyday conversation to say, "When he was a farmer, he raised lots of sheep". Hope this helps

There is no difference between "a lot of", "a great deal of" and "a large amount of" things. They mean the same things and are interchangeable. 1 sheep, 2 sheep, 20 sheep. When he was a farmer, he used to rear a lot of sheep/ many sheep. You would not say "a great deal of sheep" because you don't deal (i.e. trade) in sheep. Not for counting. Or you can say that when he was a farmer, he had big flocks of sheep. Or a great number of sheep.