A formula unit is to an ionic compound as an atom is to an element. That is, for calcium phosphate, one formula unit contains 3 atoms of calcium and 2 ions of phosphate PO4, as shown in the chemical formula Ca3(PO4)2. To determine the number of formula units given the moles, we multiply the number of moles by Avogadro's number:
12.5 moles*(6.02x10^23 formula units/mole) = 7.525 x 10^24 formula units