All of them.
It is the clothing that the players wear.
all of them do
New Era 59fifty
Depends how on the weather, I suppose.
NBA players have to wear a uniform to be recognized as to what team they belong at. It would be harder to distinguish if a team is wearing different jerseys. It is also a pride of a team to have their own uniform.
The baseball almanac website provides a list of MLB players by year they retired. They provide a comprehensive breakdown of every retired uniform number in the National League.