I don't like to think of it as how many exercises per muscle group, but rather, how many total working sets per muscle group. For each exercise I usually do 4 total working sets, however, sometimes I do 3 total sets, or even 2, depending on the exercise, where it comes in during my workout, how I feel, ect..
For larger muscle groups (thighs, back, chest, shoulders) I usually shoot for 12-15 total sets. So if I'm doing 3-4 sets per exercise (usually 4), then that's about 3-4 exercises per muscle group. I train multiple muscle groups on the same day, such as chest and back on the same day, so I'm doing around 7-9 total exercises for the entire workout.
For smaller muscle groups (triceps, biceps, forearms) I'll perform less total sets; around 8. For instance, when I'm working my biceps, I'll do 2 exercises at 4 working sets per exercise, so that's 8 total sets. I do something similar with triceps and forearms, and when I'm training my lower back.
Also, keep in mind that when you're training a muscle group, you want to recruit as many muscle fibers from the target muscles as possible. So you want to pick a number of total sets that will allow you to recruit as many muscle fibers as possible. A lot of bodybuilders say that 4 sets per exercise is the best for maximum muscle fiber recruitment for those target muscles. Any more might be overtraining (number of exercises x number of sets per exercise = total workload, which could be too high if you're doing more total sets), and any less might not be enough stimulation to recruit the amount of fibers needed to enduce maximum overload.
But bodybuilding is about trail and error. What works for me might not work for you, and vise versa. I've found that, over the years, it's a good idea to take some training advice that is proven to work, and tweak it to fit your own body's needs. I don't think I've ever followed someone else's training regimine word-for-word, without tweaking it somehow to better suit me.
Good luck with your training!