In the calcium fluoride structure the coordination number of the cation and anions are respectively

The Ca2+ ions are arranged in (ccp) arrangement, i.e. Ca2+ ions are present at all corners and at the centre of each face of the cube. The fluoride ions occupy all the tetrahedral sites. This is 8 : 4 arrangement, i.e., each Ca2+ ion is surrounded by 8F- ions and each F- ion by four Ca2+ions