Three. Each capital letter starts the symbol for a new element. The lower case "a" in Na is only there because "N" stands for nitrogen.
Disregard the subscript numbers (the 2 and 3). These only tell you the number of identical atoms of that same element in the compound.
If you are trying to determine the total number of atoms in the molecule, then use the subscripts. The molecules of this element contain six atoms in total, but only three different elements.