Oh, boy. That's a very complicated word to explain. More importantly, doing so will most definitely require a great wall of text, as even a Russian monolingual dictionary, designed for native Russians, goes a few extra miles with the word. Add to it the fact, that как doesn't really have a direct counterpart in English. Well, I need to stop complaining here.

Anyway, I'll throw in a few ideas on как много. Firstly it can be both how much and how many. As xXHoax noted, you're usually better off saying сколько instead, however, going with как много is not a tragedy if there is a good reason for it. The reason being, when you need to emphasize много specifically.

- Я получил вчера много денег.
- Как много?

Anyway, I have to go now. I'll try to expain the word better when I get back from work, if someone hasn't done so already by the time I'm here.