21, weak chin, should I get 6 or 8 mml genioplasty to look softer and feminine? (Photo)

The numbers shouldn't really matter. Hopefully your doctor provided you with a simulation so you could see your potential outcome. It would help you and the surgeon to be on the same page about what you like vs dislike. They should base movements on what you want to look like, and not just focus on the numbers. For example, one might go in thinking they will move 8mm, then realize it is too much and end up only going 6mm or 7mm. If they have not provided you with any photos or references, I would ask and make sure the surgeon understands your concerns and works with you to get the outcome you want.

An experienced surgeon will recommend the right amount of projection to fit your profile. The single photo that you provided does not show the amount of projection that you may need. In general, the minimum starting point for a genioplasty in my practice is 5mm. The soft tissue changes are not 1:1 with the bony changes. For example, moving the chin forward 5 mm may only advance the soft tissue 3 mm. For this reason, 6-9 mm advancement id often reasonable.