Pay attention to the voltage rating of the servos you choose and to the voltage rating of your RX.
Higher voltage will require lower current for the same servo power.
Then add up the stall current of the servos.
I tend to be a bit paranoid and anything above 4S LiPo gets a separate RX battery. I have seen higher power ESCs go up in smoke. Having a completely different RX power source at least lets you try to choose where it lands.
Using a separate source for the RX you should think about if you need a "power expander" or not. A power expander is a board to help you deliver current to many servos without using as much low current capacity wiring. A normal servo wire isn't good for more than 5 amps continuous demand. Heavy duty servo wires are only good for 7 to 10 amps depending on length and quality. similarly a common RX switch or heavy duty RX switch can prevent you from delivering adequate RX and servo power.
What setup you will need or want to use depends on the size of your model and its demands for the way you want to fly it.
There are no good rules/guides on this yet because people have not bothered gathering the data. I did prove that with 2 X 3200 mah 5 cell NiMh packs feeding power to a model's RX through 2 "heavy Duty" switch harnesses you can pull the voltage at the RX down to the level where the RX reboots with just six 168 in-oz digital servos by "stirring the sticks"... not even flying.
The way you test your RX system's duration is by flying. Do an appx 10 min flight and measure what it takes to recharge the RX pack. Continue checking your actual demand and adjusting your expected total flying time available, since this does change as you become more aggressive after getting used to a model. Then only fly for 50% to 75% of the time calculated to give a large safety margin. (also you are more likely to cause a low voltage issue when the pack is discharged below 20% capacity remaining)
Some of the newer radios with telemetry can have a low RX voltage alarm set. I have mine set above the minimum resting voltage for the RX pack (way above where LVC would be set if used for the aircraft's motor battery)