The baseball we watch on television or at the stadiums may be a standard nine innings (except, of course, in the event of extra innings), but this wasn’t always the case. Before 1856, the game was played until the scores of the two teams equaled 21. The idea of limiting the number of innings didn’t come about until a tied game resulted in sixteen innings.
Originally, members of the New York Knickerbockers, who set the standards and rules, wanted to have seven players on the field and seven innings. This idea was vetoed, and it was decided that the game of baseball would have nine innings and nine players on the field, just as we have today.