Infinity scarf

So I’m making an infinity scarf on size 16 round needles. I’m 5,2. How many stitches should I use? Thanks!!

By round needles, I think you mean circulars? What’s your gauge, how long do you want it? Are you working in the round or do you want to work flat and join the ends?

I do mean circular needles. The gauge is actually 16 inches, with size 15 needles. I’m knitting on the round.

Your needles are 16 in. long? Gauge is st/in. or measured over typically 4".
Several factors are involved. I’m reading that your needles are sz. 15, 16" long. The length of the needles determine how many stitches you can work and that will vary with gauge. The yarn you are using will affect gauge, thinner yarn will most likely give you more sts/in. than thick yarn. Are you planning to use pattern stitches? Gauge will not be the same as for plain stockinette. Do you have a pattern in mind? If you’re intending to use all stockinette, remember that stockinette curls.

You could cast on as many as your needles will comfortably hold and make it that long but to have an idea of how long that will be you need to know your gauge/stitches per inch. You include that you’re 5’2" so I’m thinking you want to make it longer than 16" needles will accommodate.

You can also cast-on for the width of the scarf then just knit to make it as long as you want and sew the ends together to make a big circle. I’m doing one right now that is a one row scarf so that it is reversible. They’re really popular right now so free patterns are easy to find.

Size 15 needles would either require chunky yarn - which means a bulky scarf or you can use thinner yarn and make more of a lace pattern.

To determine how many stitches you want you’ll need to knit a guage swatch. Knit about 20 stitches and about 10 rows. Then measure how many stitches you have in an inch. Then just decide how long you want your scarf (most infinity patterns I’ve read are about 50") If you want 50" and say you have 3 stitches per inch then cast on 150 stitches - or there about depending on what pattern you’re using.