This is a fun thought experiment! Here's how we can approach it:

Assumptions:

* Standard Penny: We'll use the standard US penny, which is 1.52 mm thick.

* Mount Everest: The highest mountain on Earth, with a height of 8,848.86 meters.

Calculations:

1. Convert Everest to millimeters: 8,848.86 meters * 1000 mm/meter = 8,848,860 mm

2. Divide Everest height by penny thickness: 8,848,860 mm / 1.52 mm/penny = 5,819,776 pennies

Answer:

Theoretically, you would need approximately 5,819,776 pennies stacked on top of each other to reach the height of Mount Everest.

Of course, this is just a theoretical calculation! In reality, such a stack would be incredibly unstable and would likely collapse under its own weight.