You need to work with WATTS, instead of volts and amps.
Your voltage will always be fixed at 120 volts so it is a simple formula to calculate what you will need.
Power(WATTS) = Volts(120) x Amps
Let's assume that your coffee maker uses 1200 watts, that would mean that 1200 watts = 120 volts X 10 Amps
All appliances are marked with the power (WATTS) that they use and if you use the constant 120 volts you can always tell how many amps your appliance will use. Adding all of the Amps up you can tell if you need a 30 or a 50 amp outlet.