multiply the number of chainrings (front) by the number of cogs (rear). 2x9=18.
The important thing is not how many, but what range. Your cassette's 23 tooth low gear is not very low, and the 12 is not very high. If you live somewhere with a lot of long or steep hills, you might want a different cassette. (esp. with a name like 'hillcrawler'
) If you live somewhere pretty flat you are good.
Cassettes can be inexpensive and are very easy to change.