What is the ratio between 100 Pascals and 100 mm^2?

The Pascal is a unit of pressure = one newton per square meter

the square millimeter (mm^2) is a unit of area.

I'm guessing that you want to know the pressure of one newton over 100 square millimeters.

1 square meter = 1,000 mm by 1,000 mm = 1,000,000 square mm.

So the area would be 100 / 1,000,000 = 1 / 10,000 or

one ten thousandth of a square meter.

That makes the pressure equal to. 0001 newtons / square meter.

You wanted to know the ratio?

One newton per square meter has 10,000 times the pressure than if it were just one newton over 100 square millimeters.