SELECT count(*) FROM architect WHERE id NOT IN ( SELECT architect_id FROM mill WHERE built_year  <  1850 );
How many architects haven't built a mill before year 1850?
[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);