I am pretty new to electrics. What I have been told is:

If you are using BRUSHED motors, you can use one ESC - just make sure the amp rating is equal (or more) than TWICE what you will use PER MOTOR. ie each motor draws 10 amps, the ESC should be 20 amp (or more).

If you are using BRUSHLESS motors, you will need an ESC for each motor.