Elisa, like Belle said, there's no right or wrong answer to this question.
I personally do a five-day split of back, shoulders, legs, arms (bi's and tri's), and chest in a week. Typically I do 5 to 6 exercises, 3 - 4 sets of each. That adds up to a total of between 15 and 24 sets; my average is probably 18 - 20 sets. I tend to work in the 8 - 12 rep range and get done in about an hour.
The number of exercises per muscle group is going to vary depending on the kind of split you do. The more muscles you work out per session, the fewer exercises per group you'll do. You'll probably end up with the same total number of exercises and sets, but they'll be split up among different body parts. For example, let's say you're doing back and bi's - you wouldn't do as many exercises per muscle group as I do since you'd be doing twice as many muscle groups in one workout. I think a good rule of thumb is that you want to be done in an hour or less and for me that's about 18 - 20 sets.
I think I'm the oddball here, but I try to do a different workout every time. There aren't that many different exercises so I change the equipment (DBs, BB, cables, machines), hand positions, bench positions, use one-arm or alternate arms etc. My absolute rule is that I'll never do the same exercise in consecutive workouts.

That being said, there are certain exercises that show up a lot in my workouts, like squats, pullups, pushups and the other great compound exercises.
Have fun trying out all the new exercises! It keeps working out fresh and exciting.