In this case, the MPR121 counts with 12 electrodes/capacitance sensing inputs in which 8 are multifunctional for LED driving and GPIO. The device also features a 13th simulated electrode which represents the simultaneous charging of all the electrodes connected together.
I am afraid there is not 16 channel version.
However, When the number of keys exceed the available sensor channels, multiplexing or logical combinations can be created.
In order to get more information, please refer to the AN3863 especially at chapter 4.1.2.
If I misunderstood your question, feel free to let me know. I will be glad to help.
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