What do I need to read 120 channels(input) of voltage (dual rail power supply range from -20v dc to +20v dc)?

[Henry_M]

Not all these signals need to be same GND. Right now it is still in developing process. There are a lots changes are on it ways. 

I am still working on these project. I decided to make my own circuit to measure voltage each channels. 

[Henrik_Volkers]

For slow monitoring, 120 Ch and this range, which is not finalized...   

I would also check a keithley or keysight DMM with multiplexer cards. 

Also depends on the number of cycles... MBF of relays (but usually better accuracy, really floating ) 

Getting the data into M$ is the small part...