Here’s how to fill in the blanks with "many" and "much," making any necessary changes to the nouns:

1. I don’t know **much** English slang.
2. Their cat doesn’t catch **many** mice.
3. How **many** students are there in your class?
4. He hasn’t done **much** travelling in the last two years.
5. The lawyers interviewed **many** witnesses to the crime.
6. We didn’t see **many** people on the beach yesterday.
7. Peter hasn’t made **much** progress in improving his English.
8. I didn’t know **much** English grammar before buying this book.
9. How **many** teeth do babies usually have when they’re born?
10. He didn’t have **much** fun when he was young.

Feel free to ask if you need any more help!