@RogueM is correct. There's a fair bit of configuration that needs to happen before the Cap11xx chips will do anything sophisticated, but it's all pretty straight forward to understand and a good, gentle dive into i2c protocols and the idea of setting/getting registers.
Are you using a specific software library to drive the chip at the moment, or were you going to poke it by hand and see what you get out of it?
This is the chip I've used on Piano HAT and Drum HAT so I know it well.
The datasheet is here: http://ww1.microchip.com/downloads/en/DeviceDoc/CAP1188%20.pdf
It looks daunting, but if you skip ahead to the register descriptions it's all pretty straight forward from there.
Looking over "Sensor Input Enable Registers" you'll see that they all default to 1 ( on ) so the contents of the register 0x21 should be 0xFF by default ( binary 0b11111111, 8 1s for the 8 inputs ).
If you skip back to "Main Control Register" ( address 0x00 ) you will see that the least significant bit is "INT" or "Interrupt". If this bit is set, then the chip has detected a touch. You should then read the touch status, and clear this bit by writing back the contents of this register anded with 0xFE: main_control_reg_value & 0b11111110
You can read the touch status from the "Sensor Input Status" register ( address 0x03 ), each bit in this register will correspond to a touch on a pad.
If you want LEDs to blink when you touch it, I believe it's sufficient to just:
i2cset 0x28 0x72 0xffWhich will write 0b11111111 to the "Sensor Input LED Linking Register" ( address 0x72 ). Each bit in this indicates that the LED state is linked to a touch on that sensor.
There's an awful lot of fine-grained control for LED settings, but I think the defaults are pretty sane.