Licenses are only needed for those devices that will be the destination you wish to mirror to. There is no license required for the sending device(s).
A few examples:
- If you want to mirror your iPad & iPhone to your Windows computer, you only need a license for your Windows computer.
- If you have a classroom with 5 iPads that need to be mirrored to AirServer on your class computer, you only need a license for your class computer.