In the diazotisation $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{NH}{}_2+\mathrm{NaNO}{}_2+\mathrm{HCl}$, the minimum number of moles of $\mathrm{HCl}$ consumed per mole of aniline according to the balanced equation $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{NH}{}_2+\mathrm{NaNO}{}_2+2\mathrm{HCl}\to \mathrm{C}{}_6\mathrm{H}{}_5\mathrm{N}{}_2{}^{+}\mathrm{Cl}{}^{-}+\mathrm{NaCl}+2\mathrm{H}{}_2\mathrm{O}$ is

One mole of HCl liberates nitrous acid from NaNO2 and the second provides the chloride for the diazonium salt (and the acid for the dehydration steps). The given balanced equation uses 2 mol HCl per mole of aniline, so the answer is 2.