When you use a Magic Chakram on a monster, the Chakram's enchantment gives it a random amount of power, between 1 and 100. Then, your own power times 3 is added to that. If the resulting number is greater than the remaining HP of the monster, the capture will be successful.
If a capture is unsuccessful, the monster will break the Chakram and continue to fight. You only lose Chakrams if the capture is unsuccessful. Successful captures will not cause you to lose a Chakram.