Yes, it'll make so much sense when you're at this point in your knitting. Cast on the new sts and continue knitting around and the new sts will get knit on the next round. If it says you have 88 sts after you cast on the sts, that's what you'll have then, before that maybe you had 86 so if the number is given after you cast on, that includes the new stitches. Everything will come together as you get there and follow the instructions. Don't overthink things.