One license makes you eligible to get one activation key tight to an unique application identifier (package name, bundle id, and so on)
Activation key aren't cross platform compatible.
Example: you have an application on Android and one with same functionalities on iOS.
Even if your package name and bundle ID are equal, you should buy two licenses.
During activation key request you should put Android as target platform for the first license, iOS for the second one.
Pricing: buying two licenses at once give you a 10% discount.