Hi there,
I’m curious about developing with Kirby and have instantiated a demo on my server to play around with it.
How do Kirby licences work for developers? If the client licences Kirby do I need to buy a second one for my dev environment?
Thanks for your advice!
Welcome to the Kirby Forum. 
Dev environments do not need a license, so your client only needs one license per site. You can read more about this in our license agreement.
In your dev environment you will see the “Buy license” button in the Panel, but the feature set and functionality of Kirby are not limited in any way.