Capacitive sensor MPR121 on Arduino Uno—how many will work?
The answer is 4 regardless of the current consumption.

You can have 4 MPR121 devices on an I2C bus (by connecting the ADDR pin to the right place to select 1 of 4 addresses), and the Uno has one I2C bus - so that means 4 devices.

You could use software I2C to add more devices on other pins, but that's slow and clunky.

But that aside, if you used something to increase the number of I2C buses available, according to the datasheet the peak current consumption is 1mA during sampling, and assuming 300mA maximum for the Arduino's 3.3V regulator, that would mean a maximum of 300 MPR121 connected to the Arduino.  Note that, if you could actually get to the Arduino amongst that tangle of wires, you'd find it getting rather hot...