1. It is a noun. A "buy" is a time when something is bought. It is kind of slangy, but acceptable usage. 2. "Drop" is a place to leave something, it could be money or drugs, legitimate or illegitimate. It is standard usage. 3. There were four other drops or buys.

It just simply means that the 'detectives' have been purchasing drugs from 'them' for weeks. It is not a very common way to phrase this. Some kind of slang in a way. I honestly haven't seen the movie in quite a while and am not entirely sure what the 'drop' means in this sentence, whether it means something like something is 'dropped off' or left behind or maybe they robbed this 'bank' they're referring to. Sorry haha. Interesting slang there. (edit: Yeah, I'm pretty sure I'm wrong about the 'bank' thing and it's still just referring to the drug situation so ignore my input on that one) When they are saying 'That makes five'. They are saying that something has happened five times. IF they actually were robbing banks (which I'm not entirely sure of the actual context here) then it would make sense if they were saying that they have robbed five banks. "That makes five banks now" It's a confusing sentence construct, I know. Think of it this way. Let's say you were selling chocolate bars. You're going up to people's doors and selling these chocolate bars and you have sold 10 chocolate bars so far. You could leave the door you were just at saying "That makes 10"