How many pillars hold the Earth up according to christianity?

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.

none that I am aware of

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.

Actually it was paganism that dreamed up that idea.

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

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

Is it 7, like Islam?