Knitting two pieces together

Do I have to cast on 16 stitches on my other needle in order to do this?
Neck and head:
Row 1: With 2 ends of sm and 1 of pe and RS facing k8 from spare needle of Right Sode of Body, then k8 from spare needle of Left Side of Body (16sts).

It doesn’t sound like you should cast on any sts. Do you have sts on holders from the Right and Left Sides of the body? You should use 8sts from those held sts.
What is the name of the pattern you’re making? Can you give us a link to the pattern?

(16 sts) just means that’s how many stitches you have on your needles when you finish the row.

I got it! Thank you!