For acne scar treatment, is there typically more improvement in the first or last few treatments, and how much % improvement can I expect after each session?

Acne scars should improve with each treatment session and personally I do expect to see a 20% improvement with each session.

That being said, other factors that have to be taken into consideration would be whether each scar type is being treated optimally with the most appropriate treatment method as each scar type do respond differently to different treatment methods, machine settings etc.