The number of formula units in a CsCl unit cell is

CsCl crystallises in BCC. Hence there are 2 atoms per unit cell i.e., one Cs ion is surrounded by eight Cl^- ions at eight corners. But each corner atom contributes
$8\times \frac{1}{8}=1 \mathrm{one} \mathrm{ion} \mathrm{i}.\mathrm{e}., \mathrm{one} {\mathrm{Cl}}^{-}$
$\therefore$One Cs and one Cl^- make one formula unit present in its unit cell.