Can anyone explain to me why boards like Nano and Pro Mini have 8 analogue inputs on headers, while Uno has 6, even though they use exactly same microcontroller?
They are all using the ATmega328, but in a different physical package: The SMD variants have 2 additional ADC inputs, the DIP package does not have these due to a lack of pins. There simply is no such thing as a DIP 30 package. Atmel could make one, but you maybe would not want it because there are no IC sockets available for them nor design templates for your PCB tool etc.
I would have never thought of that. Thanks!
in fact the Arduino has 1 ADC which is multiplexed over 6 pins to the outside.
You can add a multiplexer to A0 pin and create e.g. 16 analog lines this way.
Of course your code must be adapted to use this but that is not difficult.
1 ADC to 8 pins, only 6 brought to outside on DIP package.
I think external ADCs like MCP3208, with 8 analog inputs and SPI interface to Arduino are better; higher resolution, faster conversion speed.