ALPO4 can be broken up into AL/P/O4.
There is one AL mole, one P mole, and four O moles. Together, that is a total of six moles -- 1 + 1 + 4 = 6. You can find the moles for each section by breaking the atoms up by type. There are 4 O because the number 4 follows the O. There are none in front of AL or P, so they are singular.
