Knitting A Scarf With Thin Yarn??

Hi everyone!

I am going to knit an infinity scarf with 410 yards of extremely thin yarn, like so thin that I am using size 3 needles. The gauge is that 5.5-7.5 stitches is equal to an inch. My question is: how many stitches should I do? I want the scarf to be long enough that it can be an infinity scarf that loops one time, but I don’t want to buy another ball of yarn. If I make it too wide, then it may be too short for an infinity scarf. What would be a good width/number of stitches?

Thank you so much!!

Welcome!
That’s quite a range for the gauge. Have you made a swatch and determined your gauge?
A scarf 6"x54" will take about 425 yards of a fine yarn (#2 weight). Once you know how wide you want the scarf and what your gauge is, you can multiply sts/inch times inches and get the number of sts to cast on.
This is a very handy yardage estimator.