The complete equation for this reaction is,
Ba(NO3)2 + Na2PO4 = 2NaNO3 + BaPO4
Among the compounds present in the reaction, Barium Nitrate, Sodium Phosphate and Sodium Nitrate are soluble ionic compounds. Hence, they will completely ionize into ions. Only BaSO4 is insoluble which becomes the precipitate. Ionic equation is:
Ba2+ + 2NO3- + 2Na+ + PO42- = 2Na+ +NO3- + BaPO4
Cancel like ions,
Ba2+ + PO42- = BaPO4.
Thus, the stoichiometric coefficient of Barium ion is 1.