You'll either need to write some C++ code to to read the sensor and pass it to Sc. This takes the form of either a supercollider ugen, or a separate process which sends OSC. If your C++ is any good, you can use the following sources of inspiration/starting points, combined with the MPR121 example:
- supercollider ugen: use the Trill ugens as a starting point
- dedicate process to communicate via I2C and send OSC to Sc, use Trill-osc as a starting point.
If you don't want to code in C++ (or don't have the time to waste), then I would recommend purchasing one of the Trill devices which can do all an MPR121 can do but better and are already supported via the two methods above and don't need you to write any C++. Here's a link to the Trill touch sensors family: https://shop.bela.io/collections/trill . In particular, Trill Craft has similar functionalities to the MPR121 but with more pins (and perhaps better sensing).