why did Jordan choose # 23 to wear as his signnature number?

Answer 1

Michael Jordan once explained it in an interview:
"My favorite number had always been 45, which was my older brother Larry's number. When I got to high school I couldn't have 45 because Larry had it, so I decided to have half his number, 22 and a half, which rounded to number 23. That's the real story of how I came to wear number 23."

Answer 2

He chose 23 back in high school because he wanted to be half his brother who wore 45. Since 23 is not exactly half of 45 he chose 23 anyway.

His quote:
I selected number, 23, in order to "halve" my brother's 45 when we were backcourt teammates at Laney.