How many players are on the field for the defensive team?

How many balls result in a walk? How many outs does each team get per half-inning? How far apart are the bases on an MLB diamond? What is the distance from the pitcher's rubber to home plate in MLB? How many teams are in Major League Baseball? What are the two leagues in MLB? In what year did MLB adopt the designated hitter in both leagues? Which league first adopted the designated hitter in 1973?