Its all down to speed and precision really... Most MCU have 10 bit ADC's... If 10 bit will suffice, then the MCU can be set up to sample up to 16khz (DSP's can do a lot more).
As this is in the 8051 section I'm assuming an 8 bit micro is to be used (I haven't seen one with 24 ADC's onboard, but some have quite a few so I'll take your word).
If you use an external multiplexed device you'll need to get the data from each one.... My guess would be serially ( SPI )... Then sampling would be slower ( unless you intend to use flash) but can be more resolute.
So the question is... How resolute do you need your results? How fast do you need to sample?
Example..... My data is quite slow to change so I sample 24 times in a session, then about 8 times a second... 24x oversampling gives me adequate resolution, and displaying quick enough for the human eye....