The general rule for determining the number of total atoms in a molecular formula is to add all the explicit subscripts shown plus 1 for each atom shown without an explicit subscript. In this formula there are no such atoms, and the total is 2 + 7 or 9 atoms.
Impossible chemical formula. The 'highest' nitrogen oxide is the UNstable N2O5, dinitrogen pentoxide.
N3O7 is a chemical compound assigned the nomenclature Trinitrogen heptoxide. Compound is not naturally occurring.
Tetranitrogen heptoxide is N4O7 compound.
Nitroperoxynitrate