Name the number of stations for 15 minutes travel time
CREATE TABLE "circumvesuviana" (
  "lines" text,
  "opening" real,
  "traction" text,
  "length" text,
  "stations" real,
  "route" text,
  "stops" real,
  "travel_time" text
);
SELECT COUNT("stations") FROM "circumvesuviana" WHERE "travel_time"='15 minutes';