Sodium/natrium is a metal from first column group so it should have one 1+ charge. Phosphate ion has 3- charge. That is why there 3 natrium ion for 1 phosphate ion when this molecule is dissolved in water. The ion formula would be:
(Na)[tex]_{3} [/tex](PO[tex]_{4} [/tex]) ==> 3 Na[tex]^{1+}[/tex] + PO[tex] _{4} ^{3-}[/tex]
