All of them.
It is the clothing that the players wear.
all of them do
New Era 59fifty
Mark Teixeira
BARRY BONDS
Pablo Sandoval
Mankini's
Alex Rodriguez
Sixty percent
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.