The more EXs there are in the set, the more likely you'll pull one. In the last set with only 3 EXs, there was only 1 EX per box, and sometimes you didn't even get 1. The *'s have always averaged about 1 every 3 boxes (with 3 *'s per set). If they ever release just 1 * in a set, it will be astronomically rare (1 per 9 boxes, or less than 1 per a 6-box case)