You cast 40 stitches in circular needles. Or you can cast on 40 stitches; 10 on each dpn knitting needle. Dpn stands for double pointed needle. This is based on #4 yarn and 7mm knitting needles.
I prefer to use dpn knitting needles over circular needles. Sometimes circular knitting needles can give you a twisted knit.
I like to start creating the trim first. I like knitting a rib stitch; you can knit the desired width.
I am getting off topic. You wanted to know how many stitches to cast.
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