the extra gear is in the rear. you have 3 gears in front and 8 in back for a total of 24. When you hear of a "9 speed", it's refering to 9 in back. A triple (3) in front with 9 in back would (obviously) be a 27 speed.

My double 9 speed gives me 18 possible combinations.

If you find yourself "spinning" out of your highest gear a lot, you can put a smaller cassette in the rear. Smaller gears in the rear give you a faster top speed, although they are also harder to push. Do you know the size of your cassette now? If not, just go out and count the number of teeth in the smallest cog and the largest... let me know what the size is.