I’m a beginner so I know very little about multiple of. So I would really appreciate if someone can explain to me what multiple of 16 stitches +1 means? Thank you! 
Multiple of 16 + 1 means that, for the pattern you’re wanting to stitch, you’ll need to cast on 17 stitches or 33 stitches or 49 stitches or …
See the…pattern? 
116 + 1 = 17 sts
216 + 1 = 33 sts
3*16 + 1 = 49 sts
etc.
This works for any “multiple of __” plus __ sts. If you need, for instance, a multiple of 8 + 3, those numbers would be
18 + 3 = 11 sts
28 + 3 = 19 sts
3*8 + 3 = 27 sts, and just keep adding 8 each time
DCM