QUESTION: 
How many total volcanoes have erupted on Mars?

ANSWER from Jeff Plescia on January 31, 1997:
There are about a dozen really large volcanoes and an unknown
number of smallers one. The largest, Olympus Mons, has a summit
elevation of 25 km above the mean martian level and a diameter
at the base of about 600 km. If you took all of the lava from
the Hawaiian island chain and piled it up, it would be equivalent
to Olympus Mons.

There are two large volcanic provinces: Tharsis which has four
giant volcanoes (Olympus Mons, Arsia Mons, Ascreus Mons, and
Pavonis Mons), and Elysium which has three large volcanoes
(Elysium Mons, Hecates Tholus, Albor Tholus).

The ages of the martian volcanoes are not well determined.
They date from billions of years old to a few hundred million.
Given the uncertainty, they could be active today.

ANSWER from Bruce Jakosky on May 1, 1997:
Might as well go with the biggest mountain on Mars--Olympus Mons.  It's
a volcano, very similar in the style of eruption to the volcanic island
of Hawaii on the Earth.  It has the shape of a broad, flat dome.  It stands
to a height of about 25 km above the surrounding plains, and it spans
about 600 km across at its base.  This is several times the height of
Hawaii (when its height is measured from the ocean floor), so it is a much
larger volcano.

The biggest mountains on Mars are volcanic.  There are very few mountains
similar to the Rocky Mountains or the Himalayas on Earth.  The processes that
formed those mountains don't operate on Mars.