I've made 8 button touch sensors using very basic AVRs (ATTiny26) that work well with software polling. They work through up to 3mm of glass or acrylic. I can't see any reason that couldn't be expanded to any number of inputs, assuming enough GPIO.
The basic principle I used was to have a 1M pullup resistor to the Vcc on each pad. I'd then pull the pad low, and time how long it took for the voltage to raise over the trip point. Then, I counted the number of triggers and set a variable to match that time. The "calibration" variable was integrated over a few seconds so it would self calibrate and it there were a large number of low readings in a row it indicated the presence of a finger.
The only down side is that if you hold a finger on a pad for several seconds, it would be calibrated out and no longer trigger until you removed your finger for a couple of seconds.
In practice this works fine for several hundred units in us in public environments.