Final answer:
The specific number of times Hagrid knocked on Hogwarts' castle door is not explicitly stated in the books. However, there are a few instances where the number of knocks is mentioned.
Step-by-step explanation:
In Harry Potter and the Sorcerer's Stone, Hagrid knocks three times on the door to enter the castle with Harry and the other first-year students.
In Harry Potter and the Chamber of Secrets, Hagrid knocks twice on the door to enter the castle with Harry and Ron.
In Harry Potter and the Prisoner of Azkaban, Hagrid knocks once on the door to enter the castle with Harry.
Based on these instances, it is safe to assume that Hagrid typically knocks three times on the door when entering the castle with a group of people, and once when entering alone. However, there may be other instances where he knocks a different number of times, depending on the circumstances.
Overall, the number of times Hagrid knocks on the door is not a significant detail in the Harry Potter series. It is simply a quirk of his character that adds a bit of charm to the story.