SELECT TYPE ,  count(*) FROM mill GROUP BY TYPE ORDER BY count(*) DESC LIMIT 1
What is the most common mill type, and how many are there?
[Schema (values) (types)]: | architecture |  architect : id (text) , name (text) , nationality (text) , gender (text) | bridge : architect_id (text) , id (text) , name (text) , location (text) , length_meters (text) , length_feet (number) | mill : architect_id (text) , id (text) , location (text) , name (text) , type (text) , built_year (number) , notes (number);