Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_73666 ( "Circuit" text, "Difficulty" text, "Turns" real, "Direction" text, "Setting" text, "Conditions" text, "Unlock Order" real, "Unlocks" text, "Unlocked by" text )

### Question ###
How many instances is the unlocked n/a?

### Accurate SQL ###

SELECT COUNT("Setting") FROM table_73666 WHERE "Unlocked by" = 'N/A'