CREATE TABLE table_26568 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" FLOAT)

In which series was season 18?
SELECT COUNT("No. in series") FROM table_26568 WHERE "No. in season" = '18'
SELECT COUNT("No. in series") FROM "table_26568" WHERE "No. in season" = '18'