For every millimeter there is exactly one millimeter. If you mistyped the question, there are one thousand millimeters in a meter. For that matter, even if you typed the question accurately, there...
Exactly three millimeters.
1 meter = 1000 millimeters1 millimeter = 0.001 meter
Millimetres and millimetres are the same measurement. Therefore, 0.001 millimetres is equal to 0.001 millimetres.
20 millimeters