SELECT count(*) FROM Companies
Count the number of companies.
[Schema (values) (types)]: | company_office |  buildings : id (text) , name (number) , city (text) , height (text) , stories (number) , status (number) | Companies : id (text) , name (number) , headquarters (text) , industry (text) , sales_billion (number) , profits_billion (number) , assets_billion (text) , market_value_billion (number) | Office_locations : building_id (text) , company_id (number) , move_in_year (text);