The power required depends on each circuit. An Uno with a rf24, a L298 (yuck) and a motor will need a different power than an Uno with a rotary encoder, button and small LED. 10 servos will need a lot of current. A few sensors, probably, little.
So make a list of each Uno (or Mega) and research the current and voltage requirements of each part that will be connected to that Uno (or Mega). Then add the currents for the parts up for each Uno and double the sum.