I agree with zentera that it looks like a really neat project, I agree with you that using one Pi per limb is a bit overkill. Depending on how many GPIO are required per limb (or let's say each joint in each limb), GPIO expanders or shift registers would fit the bill.
I like the SN74HC595(N) series chips. Eight extra input/output pins for three GPIO, and you can daisy chain them (I've done a maximum of four for 32 extra pins using only three GPIO)
I would suspect you'd also want some form of analog in/out as well for precise feedback. MCP3008's provide eight analog inputs, and something like the MCP4922 provide your analog outs (you could also use a digital potentiometer, or an Arduino via I2C/SPI/Serial). Even with all of that connected, on a Pi3, there will still be GPIO and cycles to spare.