Body-centred cubic is a unit cell structure with one atom in the middle of a 'cube' and 8 other atoms at the vertices of the cube. However, we say that the unit cell is made up of just 2 atoms because the 8 atoms on the vertices are each shared by eight unit cells in total, so a giant structure with 1,000,000,000,000 unit cells would only have (roughly) 2,000,000,000,000 atoms (though there would be some additional atoms around the outside surface that are in fewer than 8 unit cells, but at really large volumes this becomes negligible).
Face-centred cubic unit cell has no atom in the centre, just eight atoms at the vertices of the cube and one atom on each face (there are 6 faces to a cube, so 6 atoms on faces). The atoms on each face are shared by two unit cells each, so really there are only 8*(1/8) + 6*(1/2) = 1 + 3 = 4 atoms per unit cell in a giant structure.