It is not only a matter of how buttons are arranged. BTW, they are arranged daisy chain, not in parallel. Whether this ADC can discriminate between single or multiple buttons is a matter of voltage resolution. Lets take a look at some voltage combinations. It may be read on pin PA5 on microcontroller chip.
Some buttons however exhibit curious behavior.
We can conclude that voltages in most cases are able to discriminate by the ADC to detect any combination, with some exceptions. Curiously these pins hold its values until all buttons are released. So by the side of ADC microcontroller alone, there is no reason that may prevent to detect several buttons at once. Then it signals the interrupt, supposedly through pins PB2 or PD6. Then the ISR routine in the CPU external ROM space reads from the microcontroller the data corresponding to the specific interrupt.