Hi Heinz,
You need to start by identifying which device is which. ie you need a mechanism for deciding if this is a "mobile license" or a "desktop license". You also need something you can reliable detect. (Screen-size is the obvious one, but alas that's the one thing you can't detect.)
Personally I think maybe your licensing scheme is bound to fail at some point. As the line between desktop and mobile grows ever blurrier you are going to find it harder and harder to differentiate.
Anyway, decide what it is you want to count, what it is you can actually detect, and then go ahead and count that.
cheers
Bruce