TL;DR
Likely any single LED will work and take about 20mA of current.
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Looking at my motherboard I did a quick measurement on the switch I've got connected.
without the LED connected I'm getting 5V out if it.
With it connected I'm getting 2.7V
It might be different from board to board, but it seems a simple constant current source circuit is used.
Likely maxing out at 20mA.
So regardless of what's connected the voltage is lowered to the point that the LED is using 20mA and the corresponding voltage required to reach 20mA.
based on that using a 12V LED will either not work or be extremely dim.
Connecting a piece of 12V LED strip at least doesn't light up on my end.
Anything under 5V is guaranteed to work, anything over it might not/be dimmer than expected.
