If you want accuracy, clamp 'em all on a mill bed, spot drill with a
center drill, then follow through with first a 2mm drill, then the
size, using appropriate coolant. If you can tolerate a
half millimeter of wander, you can do a 20mm stack in one go.
If you want convenience, dimple with a center punch and using
that (by feel) locate the center spur of a (for instance) Whitney
hand punch. Diameter with a punch is very repeatable, and the
hole sides will be smooth and parallel.
A drill bushing in a plate placed on top of the sheetmetal will give you
the most accurate hole location. Also use a drill with short length
flutes commonly used in aircraft sheetmetal work. You will be able to
drill a number of sheets at once if you have a factory sharpened drill.
You don't say how accurate you need the hole so you will just have to
experiment on the number of sheets that will give you the accuracy you
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