For the compound shown, how many different mono-nitro products (constitutional isomers) can result from replacing a ring hydrogen by $\mathrm{NO}{}_2$? Give your answer as an integer.

The compound is 1,3-dimethylbenzene (m-xylene). Its ring hydrogens fall into three symmetry sets: C-2 (between the methyls), C-4/C-6 and C-5. Three sets give three mono-nitro isomers (2-nitro, 4-nitro and 5-nitro), of which 4-nitro is the major one.