The formula unit is the lowest whole number ratio of ions of an ionic compound. In the case of Ca(OH)2, there is one Ca2+ ion and 2 (OH)- ions. It is not possible to reduce the ratio of the two ions to a simpler form.
The molar mass of Ca(OH)2 is approximately 74 g/mole. 50 grams of Ca(OH)2 is equivalent to 50/74 = 0.67 mole. The number of particles in one mole is equal to 6.02*10^23. This gives the number of formula units in 0.67 moles as 4.067*10^23.