If we converted ALL the world's currency into pennies, how many would we have?

A penny is 1.52mm thick. 80 quadrillion would be 121.6 quadrillion mm.
The distance from the Earth to the Moon is 384.4 billion mm, so your stack of pennies would be 316,337x taller than necessary. In fact, the distance from the Earth to Mars (closest approach) is 54.6 million km, or 54.6 trillion mm, so your little penny tower would be around 2,000x long enough. Distance from Earth to Pluto is around 4.28 billion km, or 4.28 quadrillion mm, so your stack of pennies would reach past it into interstellar space.

(If I haven’t made any miscalculations in scale)