According to recent studies conducted by the Harvard/Yale School of Sports Studies and SportsStudies Unlimited (of the United States of America), approximately 42 Million people (not counting women) play baseball in the United States, in any given calendar year (this is not even considering lifetime numbers).
Not sure about softball, will post once new study is published in September in the Journal of Comparative Sports.
According to the studies, this rate of participation gives baseball 2nd place among American sports by participation, eclipsed only by soccer. The 3rd, 4th, and 5th most popular sports are basketball, tennis, and golf, respectively.
This would include all little league, all other leagues, MLB, etc.