3) Would the maximum torque of each servo depend on the 'number of servos' I use? ..i.e. the power provided to each servo?Each model servo has it's own maximum torque specification. You must purchase the servos strong enough for the mechanical loads you will be giving them. And your power source must be able to supply the maximum current draw for all the servos combined. Plan on large expensive batteries, but battery selection can happen later while you build and prototype your project perhaps using AC powered DC output power supplies. Modified PC power supplies are an inexpensive source of DC power while designing, building, and testing projects.
I would have to respectfully disagree with selecting the batteries after getting something working without batteries, especially a robot.