before building legged robots you should read at least a little about degrees of freedom, that will pretty much clear how many servos you need for each leg.
for a toy robot that can turn and walk you can even start with 1 servo/leg but I think you will get good results with 4servos/leg. The problem is, 4 servos require a complex logic and you must be able to program that.
Also, the difference between a servo and a geared dc motor is that with the servo you can specify the position for each motor. With a geared DC you can only make it turn.