How many moles are in Na3PO4?

As given 'Na3PO4' it is impossible to tell, because you also need the mass of sodium phosphate. Then use the eq'n

moles = mass(g) / Mr

From the Periodic Table we can calculate the Mr.

Naa x 3 = 23 x 3 = 69

P x 1 = 31 x 1 = 31

O x 4 = 16 x 4 = 64

69 + 31 + 64 = 164

164 is the Relative molecular mass (Mr) of sodium phosphate.

However,

mass(g) = 1 mole X 164(Mr)

mass of 1 mole = 164 grams.

Similarly

mass(g) = 2 moles X 164(Mr)

mass of 2 moles = 328 grams.

et seq.,

So you need to know the mass in order to calculate the moles.