I want a design suiable for robotic arm and I want to know how many motors for the design?
Depends on its use. For standard civilian use, I would recommend 5 motors, as a 5-axis robotic arm.
I would recommend reading up on how to design a robotic arm on this page: http://www.societyofrobots.com/robot_arm_tutorial.shtml
Remember, each Degree of Freedom, or DOF, equals 1 more motor and encoder.