O=C(Nc1ccccc1)c1nn(-c2cc(S(=O)(=O)[O-])c([N+](=O)[O-])cc2Cl)[n+](-c2cc(S(=O)(=O)[O-])c([N+](=O)[O-])cc2Cl)n1.[Na+]
Hydrogen bond acceptor is the chemical compound which contains the lone electron pairs that participate in the hydrogen bond formation. How many hydrogen bond acceptors does the molecule contain?