The number of atoms in one BCC unit cell is ____ ?

In BCC, in addition to the corners, one atoms is placed at the centre of a cube.

The ones at the corners contribute one by eight part of it and the one at the centre is totally inside an unit cell.

Therefore, effectively inside an unit BCC, total number of atoms $$\begin{gathered} = \frac{1}{8}\times 8 + 1 \\ = 2 \end{gathered}$$