STYLETHREAD -- LET'S TALK SHOP!

I wouldn't focus on a number of jumps, rather the time and effort involved.  With any cardio workout you want to get your heart rate up and keep it there for at least 20 - 30 minutes.  You should aim for a minimum of thirty minutes, but when starting out you might only be able to jump rope for less than ten minutes. 

Get sweating, that is a really good indicator of a good workout.

ETA: After you have gotten used to jumping rope, then you may want to start counting jumps to make a game of it and help improve over time (more jumps in fewer time).