How many post offices are there in New York?
"New York" refers to state = 'NY' and name = 'New York'; 'Post Office' is the type
SELECT COUNT(DISTINCT T2.zip_code) FROM state AS T1 INNER JOIN zip_data AS T2 ON T1.abbreviation = T2.state WHERE T1.abbreviation = 'NY' AND T2.type = 'Post Office'