In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion ${\mathrm{Ca}}^{2+}$and fluoride ion  ${\mathrm{F}}^{-}$ are

In $Ca{F}_2$, having fluoride structure, coordination of $C{a}^{2+}=8$ and that of $F^{-}=4$ Each $C{a}^{2+}$ ion is surrounded by $8F^{-}$ ions has a Co-ordination number of 8 where as each $F^{-}$ ion is surrounded by ions. Thus this structure has 8 : 4 co-ordination. Each unit cell contain $4C{a}^{2+}$ ions and $8F^{-}$ions. This may be explained as follows:

$C{a}^{2+}ions per unit cell=8\times 1/8(from corners)+6\times 1/2(from face centers)=1+3=4$