White 5mm 15deg mega bright LED
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The first two numbers are the diameter so 12mm or 20mm and the last two numbers are the height of the battery in tenths of millimeters so 2mm or 1.2mm or 3.2mm.
All the batteries have the same voltage 3V so any would work but the bigger batteries hold more charge. CR2032 would light them for the longest.
You should wire the LEDs in parallel.