1. I found an article that says butterflies have 30 or 31 chromosomes; but it's an old article from 1978, and scientists have learned a lot of new things since then.
2. Here's a link that says butterflies have about 380 chromosomes and a huge genome, 40 times bigger than ours:genetics
3. Somebody wrote a book called "DNA and Heredity" that says butterflies have 380 chromosomes.
4. I found the best article yet in Nature magazine, which is a very famous science magazine. It says that there's one group of butterflies where the numbers of chromosomes in different species vary from 20 to 268. That article is from 2005, which was not so long ago.
So, I think the number 380 is a mistake, because I can't find that number in any scientific articles. I think there is a big variation in how many chromosomes a butterfly can have, like it says in the Nature article.
So I think you and your teacher are both wrong! Butterflies don't have ~380 chromosomes, and they don't all have 20-40 chromosomes either!
One thing you need to remember about counting chromosomes is that you can count them in 2 different ways to get 2 different numbers - if you count the chromosomes in eggs or sperm, humans have 23 chromosomes. That's called the 'haploid' number. But after the egg and the sperm get together, there are 46 chromosomes, which is 23 pairs.
It was hard to find the answer to your question, and I got the best answers by searching the google scholar site.Best wishes,