The A320 has slide rafts on the main doors that are designed to carry passengers in the event of a water landing i.e. they float and detach from the aircraft. In the event of a land evacuation they function as slides. The overwing exit slides are not designed to float or be detached and normally would only be used for land evacuation.
Separate life rafts are also carried which need to be taken out of their compartments and manually launched.
Unlike the Titanic, there are more than enough places for everyone onboard and some overload capacity beyond the stated numbers is built in just incase.