One thing to remember about the quality graphs in AS is that it's only relative to the "best" quality frame in the video, not an absolute quality figure. So if you capture 1000000 evenly terrible frames, the quality graph will look great, but the image will look terrible because they are all bad, just equally bad.
Stacking less images might lead to a sharper result, however it will most likely be more noisy and so you wont be able to apply the same amount of sharpening to the final stacked image and so might end up looking worse.
As I mentioned in another thread, with my C9.25 and ASI224MC, my normal process is to image Jupiter for 3 minutes at 150 fps (so 27000 frames) from which I normally use stacks of 3000, 5000 or 7500 frames (seeing dependent) and process which one is better. For Saturn I use 5 minutes at 100 fps (so 30000 frames total), and stack 5000 to 10000 frames. For Mars I use 4 minutes at 250 fps (60000 frames) and stack 3000 - 5000. Neptune and Uranus I image for 5 minutes at 20 fps, normally stack about 50% of them.
I recently tried using WinJupos to derotate longer videos for Jupiter and Saturn, my latest effort was a 15 minute video on Saturn (see image section below). Since I'm on an alt/az mount I was able to use WJ to remove both field rotation and planetary rotation on the video, then stack and debayer in AS!3. Here I was able to stack 10000 frames out of 90000 to get very low noise levels but also able to use high amounts of sharpening without a huge amount of denoise in Registax, so the final result was sharper.
Here is a video showing an interview with Emil about the soon-to-be-released AS!4.
https://www.youtube.com/watch?v=JIjXmRh1DE0
Andrew