decreasing
Now for the top, you'll be using the final technique you'll have to learn: decreasing. Decreasing is just as easy as increasing, however it is a little bit more difficult to explain.
Now it's time for some decreasing action.

1. You're going to start by doing all the double crocheting steps you usually would do up to the point where you have two loops on your needle. So, chain three, hook yarn, insert in butt, etc. stopping with the two loops on your needle.
2. With the two loops still on your needle, start the next stitch. Hook the yarn and go into the next butt. Hook the yarn again and pull it through. You should have four loops on your needle.
3. Hook the yarn again and pull it through two of the loops.
4. Hook again and pull through the remaining three loops.
5. Set down your work. Go to the nearest window. Open it and proclaim your status as decreasing master. Bow regally.

You can play with your top in terms of how much you'd like to decrease and where. If it's decreasing too rapidly, you can always break from it and go back to the double crochet. You could also decrease only on one side so that your triangle is straight on the arm side, which will offer more coverage.

As far as how many rows, the best part about decreasing is that it dictates how far you can go; when you've decreased down to zero stitches, there's simply no more work to be done. On my top there are about twenty-five rows. If you have a larger bust you may have a lot more.