One. A hydrogen ion (H+) has a +1 charge, and a chloride ion has a -1 charge, Cl-, so they combine in a 1-to-1 ratio to form a neutral compound, HCl or hydrochloric acid.
Chlorine is number 17 in the periodic table, so using the electron shell arrangement, 2:8:7: It has 7 electrons in its valance shell so its charge becomes Cl7+ when these 7 electrons are 'donated' (=...