It’s a way to twist your knit stitch, to make it fancy, you could do it on any multiple of stitches, it’s up to you. It’s usually used in ribbing though.
it can be done on any number of stitches. Sometimes to add a little texture to plain knittting I k1tbl on all stitches then purl as normal thi also gives a nice look. If your’e feeling brave kttbl on knit rows and ptbl on purl rows. Try a square see what you think and what suits you.