You can really add as many as you want. If you're nervous about sewing and cutting, you can add more. If you're doing a Norwegian steek, you don't add any. I add for Norwegian steeks.
I did 10 with very thin yarn. As long as you have room to run rows of stitches down, or crochet the steek if you choose to go that way, it really doesn't matter. You do, however, knit them every round, so you don't want to add too many.