1. The amount of popcorn kernels you use depends on what kind of popcorn you want. If you want popcorn that's completely covered in caramel (like the second photo in this post), use ½ cup kernels. If you want popcorn with bits of caramel here and there, with some white still showing (like the first photo in this post), go for the ¾ cup.
2. Caramel popcorn technique adapted from
The Kitchn.