How many pillars hold the Earth up according to christianity?

None. You never heard a Christian teach that. The Christians teaching that every word of the Bible is inspired have never read the Bible or they wouldnt say that

none that I am aware of

It's a myth. There are no pillars.

NONE...GOD HUNG THE WORLD ON NOTHING! THAT IS POWER!! THE WIND AND THE SEA OBEY HIM ALSO.

None...................

How many do you see?

Job 26:7  "He spreads the skies over unformed space, hangs the earth out in empty space."

Apollo 17 lunar mission, Astronaut Eugene Cernan, on his second flight to the vicinity of the moon.   “When you look back at the earth from the moon and you see the perfectness of it and the beauty of it and the logic of it all, you know it didn’t happen by accident. It is moving with beauty and you get a feeling that you are looking at our earth as God, whoever that God might be, envisioned it when he created it. I’m anxious to get back and get that feeling again.”—New York “Times,” December 8, 1972. 

None. The earth is held by the gravitational pull of the sun in its orbit, and the other planets in the Solar system keep it where it is supposed to be, for they are required to give gravitational pull in its rotation around the sun.

The pillars are metaphorical concepts.

According to Job 26:7 it shows that God stretches out the northern sky over the empty sky & suspends the earth upon nothing. So there are no pillars of Christianity that hold the earth up.

In the Bible
the only person who claims that the earth (NOT capitalized) is held up by pillars
is Hannah
and that: in an impromptu song.

So:
- not God
- not an angel
- not a prophet
- just some woman
- who was making up lyrics to a song on-the-spot

So: typically Christians answer: "None, the Earth is not held up by pillars."

One, it's called the Holy Spirit. When that leaves everybody dies apart from a handful.

Next question …..

God told us that the earth hangs in the sky on NOTHING.  God doesn't need pillars to support what He makes.  He merely speaks and things come into existence. That's how we know there is a God in heaven - a wonderful Creator because no man could have made all the beautiful things God has created. Mankind can't even replicate a blade of grass without a seed first and man cannot replicate life, only God can.

We can resuscitate where life ALREADY existed, but no human being can produce life in and of itself.  They have to have an egg, a seed, or a fruit containing seeds to start life

None, the Earth is held in place by an equilibrium of gravity and centrifugal force.

I suppose minimum number would be 4, as "everybody knows" there are 4 corners to the world

Is it 7, like Islam?

Actually it was paganism that dreamed up that idea.