It actually can mean both depending on the situation. It's a colloquial phrase that tends to be used in the plural, but again it really just matters in terms of what is being referrenced.

"That stuff" refers to the collective knowledge that you have about something. That knowledge may cover more than one area of expertise. It depends on the context. If someone says to you, "How do you know about that stuff?", that means you probably impressed or surprised them with what you know.

It can be one or more.