What is the coordination number of any particle in the $fcc$ (face-centered cubic) crystal lattice?

What is the value of the radius ratio for an ionic crystal having a coordination number of $6$?

What is the coordination number of a cation in an ionic compound if the type of hole occupied by the cation is octahedral?

The number of atoms in body-centred and face-centred cubic unit cells are, respectively:

$(i)$ What is meant by the term 'coordination number'?
$(ii)$ What is the coordination number of atoms:
$(a)$ in a cubic close-packed structure?
$(b)$ in a body-centred cubic structure?

The $FCC$ unit cell of a compound contains ions of $A$ at the corners and ions of $B$ at the centre of each face. What is the formula of the compound?