Stupefy is a decently strong spell meant to temporarily neutralize a person but to not cause any severe harm. However, Stupefy can be very strong, especially when more than one spell is targeted at the same person, as in the case of Professor McGonagall. Professor McGonagall was attacked with a volley of Stupefy spells by several wizards from the Ministry of Magic, and she dropped unconscious and had to be taken to the Hospital Wing.