Eight. If eight of the 13 are red, he will have eleven total red candies (when he adds the three he already had). The remaining additional candies (only five are left) would be green. These would combine with the four green that he already had, giving him nine total that were green.
If he had picked seven red and six green, that would have left him with ten of each - thus not fulfilling the requirements. The eleven/nine answer meets the "least number" requirement.
The 25 red/25 green part is a standard example of useless information designed to confound the reader.