Yesterday was Jackie Robinson day across Major League Baseball. Let’s be clear on one thing – this is a yearly event that baseball just knocks out of the park (pun totally intended). There are a lot of great events, dedications, videos, etc. all commemorating the anniversary of the day Robinson broke the color barrier (well the modern color barrier, this guy or this guy were probably the first African American player depending on your definition of “major” league). At first blush it is odd to think about why a rules/umpire blog would discuss this. Well, one of the things MLB does on this day is all players wear #42. I was watching MLB Tonight and during their tweet answering segment (they had a better name for it), someone asked how umpires handled all the players having the same number. While I am not 100% sure, I think the answer is buried in the rule book. Here is the first sentence of Rule 3.06 Comment:
To avoid any confusion, the manager should give the name of the substitute, his position in the batting order and his position on the field.
Rule 3.06 deals with substituting. Notice the comment states the manager should give the name of the substitute. In other words, the number does not matter – they are just a courtesy to help umpires, PA announcers, fans, and others to keep up with things. Here is a clip of Jonathan Papelbon wearing the wrong number in a spring training game. 
Obviously the umpires let him in the game. On the lineup card they listed Papelbon in the game – not Ruiz, not #51 but Papelbon. Rule 1.11 (a) (1) now lists that all players have to be on the field with numbers. This was not always the case. While not new, the practice of wearing numbers is not as old as baseball itself. It was not until the 1930s that all teams wore numbers. That means the last Cubs World Series winning team did not wear numbers. The 1929 Indians and Yankees were the first team to go beyond experimentation and wear them all the time. The Yankees assigned their numbers based on position in the batting order. This is why Ruth was #3 and Gehrig was #4. You can read more on the history of numbers via the wikis. Uniforms are on page 11 and substitutions are on page 16 of RuleGraphics
BetterRuleBook.com 