Answer
The answer is 9. Van Nistelrooy wore the number 9 jersey during his time at Manchester United, from 2001 to 2006. He is widely regarded as one of the club's most prolific strikers, having scored 150 goals in 219 appearances. Van Nistelrooy's goal-scoring prowess and clinical finishing made him a formidable force in front of goal, contributing significantly to Manchester United's success during that period.