Not every robotic arm is the same. They have different load ratings, that is, the maximum load that the end effector can take, the speed and reach, that is, how far the end effector can reach. However, one very important part of a robotic arm is the number of motors it has. So, for every axis, you need at least one motor to make the robot traverse in that axis. For example, a human arm has three-dimensional freedom in the shoulder joint. Hence, to mimic that joint, you will need a motor for every axis, that is, a minimum of three motors are required for the arm to move in all the three axis, independently. Similarly, when we talk about the elbow joint of our hand, it can only traverse in two dimensions. That is the closing and opening of the arm and the finally the rotation of the arm—the elbow does not move in the third dimension. Hence, to replicate its motion, we need at least two motors, so that we an move the robot in the w axis.
From what we have understood so far...