RP2040 has five ADC channels in total, four of which are 12-bit SAR based ADCs: GP26, GP27, GP28 and GP29. The input signal for ADC0, ADC1, ADC2 and ADC3 can be connected with GP26, GP27, GP28, GP29 respectively (On Pico board, GP29 is connected to VSYS). The standard ADC range is 0-3.3V. The fifth channel is connected to the in-built temperature sensor and can be used for measuring the temperature.

Use the machine.ADC class:

from machine import ADC, Pin
adc = ADC(Pin(26))     # create ADC object on ADC pin
adc.read_u16()         # read value, 0-65535 across voltage range 0.0v - 3.3v

The argument of the constructor ADC specifies either a Pin by number, name of as Pin object, or a channel number in the range 0 - 3 or ADC.CORE_TEMP for the internal temperature sensor. If a pin is specified, the pin is initialized in high-Z mode. If a channel number is used, the pin is not initialized and configuring is left to the user code. After hard reset, RP2040 pins operate in current sink mode at about 60µA. If the pin is not otherwise configured, that may lead to wrong ADC readings.