How many different total number of transit passengers are there in London Luton?
CREATE TABLE "2008_data" (
  "rank" real,
  "airport" text,
  "total_passengers" real,
  "pct_change_2007_2008" text,
  "international_passengers" real,
  "domestic_passengers" real,
  "transit_passengers" real,
  "aircraft_movements" real,
  "freight_metric_tonnes" real
);
SELECT COUNT("transit_passengers") FROM "2008_data" WHERE "airport"='London Luton';