I need help to create billboards

Hi all,
In my mobile game;
I have only one scene, everything except the characters are static, including the camera.
Therefore instead of the static 3d models I want to use picture of them.
the problem is because this is a mobile game I don’t know the effect of the different resolutions.
How can I solve this problem?

Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:

https://discussions.unity.com/t/845782/4

https://discussions.unity.com/t/848795/5

the thing is they are billboards. They are just sprites in the scene not in UI.

This covers the relationships between PPU on sprites and camera size and screen pixels and whatnot: