how can I increase the number of Loop Cuts with my Apple Magic Mouse. Normally it is with the scroll wheel, but that does not work with the Magic Mouse. And I know that I can change the number of cuts afterwards, but sometimes the cuts then are unproportional. Even if I set the cut right in the middle by clicking the left mouse button.
You can use the keyboard (both the top row and the numpad work) to type the number of cuts.
The current number of cuts is displayed in the Header, where you can also see the result of modal numinput expressions (modal numinput will be added in 2.7, and is already in development versions):
This doesn't give you smooth scroll control, but if you still want to control the loop cuts from the Magic Mouse, you can use the MagicPrefs app (http://magicprefs.com) to map One Finger Tap Left to the + key and One Finger Tap Right to the - key. This allows you to inc/dec the loop cuts by tapping on the magic mouse. Not as slick but I think it's better than moving your hand to the keyboard.
Additionally if the mapping causes issues in other apps, you can apply the mapping just to Blender itself in MagicPrefs.