In the NBA, certain jersey numbers are reserved or have restrictions. Specifically, the number "69" is not allowed in the NBA as it has been deemed inappropriate. Among the options you provided (23, 45, 12, 56), none of these numbers are typically against league rules.
However, if we assume Lester's request was for a number that is generally inappropriate or controversial, none of these numbers specifically violate NBA rules according to the standard guidelines.
If you meant that he requested a number associated with a player who has retired or is otherwise honored (like Michael Jordan’s 23 or Michael's later honored first number, 23), then it's a possibility but not against any explicit NBA rule.
If to interpret the coach's answer as a mistake or confusion, other numbers might fit that interpretation. In conclusion, if the problem is to identify a number that would create controversy or opposition, "23" for Michael Jordan comes to mind, but ultimately none of the numbers you provided are actually against NBA rules.