MicroPython code for the raspberry pico to count how many WS2812 are connected, the data signal of the LEDs must be connected to GP2 and the ADC (GP26) must be connected to the strip ground after a jump wire (I used a cheap 20cm jumper cable), using the jumper as a shunt

main.py

import machine, neopixel, time
from machine import Pin, ADC

np = neopixel.NeoPixel(Pin(2), 200)
adcpin = machine.ADC(26)

while True:
    num_of_leds = None
    for i in range(len(np)):
        onvalue = 0
        offvalue = 0
        for t in range(10):
            np[i] = (255, 255, 255)
            np.write()
            onvalue += adcpin.read_u16()
            np[i] = (0, 0, 0)
            np.write()
            offvalue += adcpin.read_u16()
        dist = abs(onvalue - offvalue)
        #print(f"{i+1} on: {onvalue} off: {offvalue} dist: {dist}")
        if (dist < 1100):
            num_of_leds = i
            break

    if (num_of_leds):
        print(f"Found: {num_of_leds}")
    else:
        print(f"No LEDs found.")
    time.sleep(3)