Uno and Duemilanove boards have 20 I/O lines, and can handle up to 12 servos using
the standard servo library, if I'm not mistaken. However, it is a royal pain to connect
that many servos to those bds, since servos are 3-pin devices and the usual Arduino
bds have 1-pin I/O headers.
If you really want to do a walker with Arduino you might hunt around for bds that
can interface servos easier. Others might suggest what is available - I designed my
own pcb for my projects. Another possibility is to buy a dedicated servo controller
bd where you can plug in the servos directly, then you will have at least 18 I/O
lines available on the Arduino.
Of course, doing all of this adds "weight", which is the bane of walker design.
For reference, I am currently working on a quadruped [Poco] that uses Lynxmotion
pan-tilt brackets for the legs. These are fairly heavy, and I suggest using something
lighter in weight. I started with HS322 Hitec servos, and it walked initially but after
adding a host of sensors, pan'n'tilt pod, etc, the servos were no longer powerful
enough for walking any longer. So I am currently changing over to MG946R servos.
Both kinds of servos are available for about $10/ea. The 946s came from Hobby King
in china. Of course, heavy legs, pan'n'tilt pods, sensors, etc, all add weight so then
it needs heavier batteries. Vicious cycle, building walkers.