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.