Connecting multiple signals to ADC
There are multiple different options you could use to fix this issue.  Grouping the sensors in 5 different groups is not a feasible solution unless you have multiplexers connected to each group before going to the microcontroller. 

One option is to connect a MUX 8-to-3 channel to 8 sensors and 3 microcontroller pins.  This way, you would be able to communicate with 16 sensors using 6 pins.  This is not ideal since you want to communicate with 20 sensors and 8 pins.  Just thinking about it, I came up with a solution using 3 MUXs and 1 decoder.  This way, you can assign a sensor address and control which MUX you want the data from using the decoder.  This could work but I am sure you could use multiple different ways: 







simulate this circuit &ndash; Schematic created using CircuitLab

With the schematic shown above, you can connect 24 sensors on the "D" pins of the MUXs and you only need 7 pins of your microcontroller to get the signal.  The output pins (Y and /Y) would be connected to your ADC on your microcontroller with the right sampling rate and so on.  Nothing would change there.  But what you would be reading would be dependent on what you set your decoder at.  

Another way you could achieve this is to use a I2C bus and assign an address to every sensor in your array.  This would be a bit more complicated communication/programming wise but it would be very efficient.