How many people are named Janessa? | Do you mean the number of people whose first name is Janessa? | Yes
SELECT count ( * )  FROM Addresses AS T1 JOIN Staff AS T2 ON T1.address_id  =  T2.staff_address_id WHERE T2.first_name  =  "Janessa"
number of people named Janessa refers to COUNT(*); first name is Janessa refers to WHERE first_name = 'Janessa'; joining Addresses and Staff tables refers to necessity of data from both tables to fulfill the condition;