I was confident in freehanding amigurumi after making a few from patterns and I got a feel for how many stitches I need to make whatever shapes. So maybe you can do that?
it would depend on the thickness of your yarn and how big you want the initial size to be i would think, youd just have to play around and do a lot of testing
Most patterns start with 6 single crochet in a magic ring. In the 2nd round it can go up to 8 crochet or 12, depending on what’s being made! Then usually the rule is that it goes up by intervals of 6