Lowest multiple of 12 under 50 is 48
48 /12 = 4
Lowest multiple of 12 under 100 is 96
96 / 12 = 18
4 CDs can be cut across the height, 8 CDs can be cut across the width.
Hence 8 x 4 = 32 CDs can be cut. This is assuming you are allowed to waste extra plastic or stick 2 pieces together in some way.
Diagram on next comment.
4 CDs can fit the height (50cm) of the plastic with 2 cm left over.
8 CDs can fit the width (100cm) of the plastic with 4 cm left over.
4 (in height) x 8 (in width) = 32 CDs.
The remainder of the card would not be big enough to cut out CDs from. You cannot cut more circles as you cannot stick 2 pieces of plastic together to make a CD.
Area circle = (6^2) x pi = 36pi
Area of plastic sheet = 50cm x 100cm = 5000 cm^2
5000 / 36pi = 44.21 (2dp)
32 CDs could be cut, but 44 CDs could be cut if you could stick plastic together to make a CD.