Debouncing : Can we use only one capacitor for multiple Key?
If you connect each button to a different MCU pin, then you must have a capacitor for each button.

You can also connect all buttons to a resistive ladder and use MCU ADC to sample the voltage, bounce is not a problem then, because of low speed of the ADC.

My preferred solution: don't debounce, just lower the sampling rate. Eg. have a timer interrupt between 10-100Hz and sample buttons in the main loop only when the interrupt handler sets a flag.