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_2591 ( "No. in series" real, "No. in season" real, "Title" text, "Directed by" text, "Written by" text, "Original air date" text, "Production code" text )

### Question ###
How many series numbers do the episodes originally aired on October 28, 1990?

### Accurate SQL ###

SELECT COUNT("No. in series") FROM table_2591 WHERE "Original air date" = 'October 28, 1990'