If you get subwoofers for your car do you need to have an amplifier?

Yes unless its a powered subwoofer

Match the RMS power of the amp to the sub(s). Ignore so called 'peak' and 'max' power, called fuzzy wats and just plain hype by tuners and tweakers alike. Some manufacturers also use the term 'constant' power. By keeping everything balanced you keep the components stress level down for longer life and save money. Buying a 1000 wat amp for 150 wat subs is a waste of money.