unsigned OEGetAromaticRingCount(const OEChem::OEMolBase &mol)

Returns the number of aromatic rings in a molecule as defined in [Ritchie-2009]. The article describes the method as the following:

The terminology 'number of aromatic rings' (or aromatic ring
count) is used generically and encompasses both benzenoid
aromatic rings and heteroaromatics (including, e.g. pyridine and
imidazole). ... Each ring in a fused system is counted
individually; thus, indole and naphthalene are each defined as
having two aromatic rings.