Coordination numbers of ionic and intermetallic compunds
NaCl

NaCl is not monoatomic. When discussing coordination number of cations in ionic compounds, the number refers to the anions the cations is surrounded by (or bonded to, or touching).



The blue Na+ cations are bonded to six other Cl- anions: top, bottom, left, right, forward, backwards (in this image). They are not bonded and do not touch the other Na+ cations in the diagonals.

CsCl

CsCl is not a "simple" simple cubic. Each of the different ions, Cs+ and Cl- by themselves are simple cubic, but they are interpenetrating to form something that looks like a body centered cubic:



A mono atomic simple cubic will have coordination number of six, but this is an ionic compound. Thus, each cation or anion that sits in the center of the cube is bonded to the eight anions or cations (respectively) that sit in the vertices of the cube.