so area of a circle is pi x radius squared where radius is half the diameter. You also need to work out how close the circles can get, as obviously they don't tessellate so there will be some wasted card.
alterniatavely, you could just assume that as each circle has a diameter of 60 mm it will fit in a 60 mm x 60 mm square and do the calculation from there...? (60 mm = 0.6 cm). Obviously can only have as many circles wide as there are 0.6 cm in 20 cm.
Hope that makes some sense, it's hard to articulate when I can't draw you a diagram!