How many $100 are in a stack?

Excuse me, could you clarify something for me? I'm a bit confused about the terminology used in the statement "How many $100 are in a stack?" Are we referring to physical currency notes, specifically the denomination of $100? And if so, could you elaborate on what exactly constitutes a "stack" in this context? Is it a predetermined number of bills bundled together, or does it vary depending on the situation or region? I'm genuinely curious to understand the specifics of this question.