(1) The nearest neighbour distance is half of body diagonal (a) root 3/2 (along body diagonal) thereforecoordination number for a given atom in BCC unit cell is 8.
(2)The next nearest neighbour are 6 at distance (a)Lattice parameter.
(3) 3rd neighbour (Next to Next nearest neighbour) are(12) at distance (a root 2) ( All corner along face diagonal in x,y and Z plane).