What does the word "instances" mean in this context? A case or a result, perhaps?

Does "herb" mean "cannabis" here? What are the other slang words for "cannabis"?

Yes, you are correct. "Herb" is used here as slang for marijuana. The most commonly used slang words are "weed," "pot," "grass," "ganja," "dope," and "reefer." Ex: Smoking pot is legal in many countries. Ex: I can smell weed. I bet someone's smoking nearby.

Why the word "Count Rushmore" came up all of a sudden?

While watching Jeopardy, Joey guesses the answer to be "Count Rushmore." Chandler gets upset later during his conversation with Joey when he finds out that he doesn't like Janice. Thus, he leaves the room and says out of spite, "there is no Count Rushmore!" He is making fun of Joey because the correct answer is Mount Rushmore, the national monument found in South Dakota. Count Rushmore truly doesn't exist.

What does it mean by "break you as a leader?"

To "make or break it" basically means to succeed or fail at something, so "make or break you as a leader here" means "to succeed or fail as a leader". Ex: Height won't make or break you as an athlete, but it can definitely be helpful. Ex: For children, having a healthy family environment can make or break you when they become adults.

What's the main difference between "socks" and "stockings?"

Good question! The difference mainly lies in the length. Socks are usually shorter and don't reach past one's mid-calf. Stockings are much longer and reach up to one's knees or even past that. Ex: I bought some knee-high stockings when I went shopping. Ex: My ankle socks have a hole in the toe, so I need to mend them.

What does "yellow-bellies" mean?

"Yellow-bellies" is an insult meant to call someone a coward. Ex: You yellow-bellied scoundrel! Ex: I can't believe that yellow-bellied fooled abandoned us.