how much current does an ADC input need to function correctly
The input impedance of an analog input is specified at 100 Meg Ohms, so virtually no current.
I see. 100Meg for Arduino's I assume. Do different uC have vastly different ADC input impedances?
Could be much higher than 100Megohm at room temp.
An A/D with a PGA (programmable gain amplifier) could have a much lower input impedance.
As an example, see the ADS1115 datasheet, table2 on page13. This non-ratiometric A/D is not suitable for reading a pot (as you asked in another thread). Leo..
Here Impedance is misleading. The typical audio impedance, measured at 1kHz, is meaningless with faster signal processing. While the ADC mux has high resistance, it has a significant capacity which determines the effective input current.