From the Super Mario Wiki
The Creepy Steeple Boo is a Boo from Creepy Steeple in Paper Mario: The Thousand-Year Door. He is the Boo that asks Mario "How many Boos just came out?" after Mario frees the two hundred Boos from Creepy Steeple. He will drop an Ultra Shroom if Mario guesses correctly. It is unknown if he was actually one of the Boos that joined in to make Atomic Boo, as he is presumably a friendly character throughout the game. It is unknown if he was also referring to himself when he asks how many Boos came out. Later in the game, he sends Mario an email saying that he is thankful for Mario freeing him and his friends; he also reveals in the email that there are many secrets for Mario to find in his home of Creepy Steeple. That email shows his dislike for technology and seemingly frequent criticisms of himself.