I have a weird anomaly with a WS 2811 12 v strip. I have a strip with 100 LEDs. For every one LED I add under Led Preference it lights up 3 on the strip.
eg
So if my LED preferences say 3 under Length, 9 LEDs light up.
That’s a side effect of that particular 12V strip. They combine 3 LEDs in series to get closer to 12V.
Other strings use a different controller and/or onboard regulator for each LED.
You’ll have to research what you’re buying to know for sure.