Answer:
When you're looking at the entire surface of Mars, captured from space, there are 4 huge volcanoes visible in the Tharsis region of Mars. Olympus Mons is the largest, and the other 3 are Ascraeus Mons, Pavonis Mons, and Arsia Mons. While Olympus Mons towers 24 km above the surrounding plains, and spans 500 km across, these smaller volcanoes are merely 350-450 km across and stand 15 km tall.
These are just the largest, but there are a total of 20 named volcanoes across the surface of Mars.
Credits to: Universetoday.com