With the default prescaler of 128, the maximum sample rate is
9615 samples/s (16 MHz/128/13). It can be achieved by setting the ADC to
“free running mode”. Running analogRead() in a tight loop, the maximum
rate is 8929 S/s (16 MHz/128/14). As there is only one ADC, this is the
total rate for all inputs being sampled.
is there a way to disable muxing and select which one input is active
The MUX is a physical device that sits between the inputs and the ADC. You cannot physically remove it. What do you mean by “disable”? You can, of course, keep the channel selection constant.