Learn many about

I submit the following for your consideration: (l) If you want to stress the depth of knowledge that you received, you might say: "We learned (a lot) (much) about the plants and animals," and (2) If you wish to emphasize the variety of plants and animals that were studied, you might say: "We learned about (many) (a lot of) plants and animals." Sincerely, James

(a) We learnt many about the plants and animals. No. We learnt (learned for me) about many plants and animals. We learned much about plants and animals.

(b) We learnt of / about a lot of plants and animals.

(c) We learnt many things about plants and animals.