CREATE TABLE table_2591 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT)

How many series numbers do the episodes originally aired on October 28, 1990?
SELECT COUNT("No. in series") FROM table_2591 WHERE "Original air date" = 'October 28, 1990'
SELECT COUNT("No. in series") FROM "table_2591" WHERE "Original air date" = 'October 28, 1990'