Hydrochloric acid is a mixture. Although we use HCl as the formula for hydrochloric acid, HCl is, in fact, the formula of hydrogen chloride, which is a compound. Hydrogen chloride is a gas which readily reacts with water.
HCl + H2O ———-> H3O^+(aq) and Cl^-(aq)
Hydrochloric acid is a mixture of these ions
H2O and H3O^+(aq) and Cl^-(aq)
There are many different concentrations of hydrochloric acid - it must be a mixture.