How many different monochlorination products (constitutional isomers only) can the alkane shown give with $\mathrm{Cl}{}_2$ and light?

The alkane is 2-methylpropane (isobutane). It has nine equivalent primary hydrogens (three $\mathrm{CH}{}_3$ groups) and one tertiary hydrogen, so two sets of equivalent H exist. It therefore gives two products: 1-chloro-2-methylpropane and 2-chloro-2-methylpropane.