ADC for pH and conductivity sensor
You have not specified which sensors exactly you're referencing. So I'm assuming the pH or conductivity sensor is a bare analogue sensor and not an active device.

Bit resolution is a symptom of quantising a continuous quantity. In other words, the analogue signal from your sensor has no resolution (or, if you will, can be assumed to have an infinite resolution). It's only when you try to digitise the continuous quantity that the bit resolution is defined.

The number of bits you choose 8, 12 or 16 is dependant only on the resolution you require on the digital side. But you may need to take into account other factors such as the reference voltage you will use to power the sensor.