How many atoms are centered on the [100] direction in a FCC unit cell

There are 2 atoms.

Explanation:

There are 2 atoms that are centered on the [100] direction in a FCC unit cell. Because for a [100] plane, there remain 1 atom at all of the 4 corners of cube and all of them are distributed through all of the 4 adjacent unit cells.

And the center atom also remains in the unit cell as well.

Hence, there is the equity of 2 atoms that are been with this FCC (100) plane.