Building a robot with wheels or with legs needs a design completly different. One of the biggest difference is the type and number of motors. While for a robot with wheels usually 2 or 4 servos or gear motors are enough, when we speak about a robot with legs, the number of motors increases dramatically. It is not uncommon to see a hexapod robot with 18 servos. A quadrapod robot has usually 8 or 12 servos, depending on the number of joints. Luckily, compared to robot with wheels, usually are not required servomotors with continuous rotation or gearmotors. Standard servos (0-180 degrees) are enough. Servomotors are driven with PWM and Arduino 2009 or Arduino Uno have 6 PWM pin. And this is a problem. And if you want to use also other sensors using digital pins, the situation is not good, because the Arduino’s digital pins are only 14, and 2 of wich are used for the serial port communication (pin 0 and 1). It is possible to use the Arduino Mega, with 14 PWM pin, but it is very expensive (50 euros and more). Searching in internet, I found a low cost chip, using I2C protocol that can drive 12 servos. First good news, the I2C protocol uses only the analog pins 4 and 5. Second good news is that you can use up to 8 chip, so it is possible to drive up to 8x12=96 servos with only 2 analog pins, the 4 and 5. Third good news is that the chip costs about 4 euros and half. A price that can be accepted by every robot builder.
More information, pics, code example here: http://robottini.altervista.org/12-servos-controller-low-cost
Disclaimer: I'm not payed by www.hoobytronics.co.uk, I'm not an official tester, I'm not linked to www.hobbytronics.co.uk, I'm only a robot builder that shares his experience.
https://www.youtube.com/watch?v=YoMiVgMsWQM