the “K2tog 3 times, (YO, K1) 6 times, (K2tog) 3 times” is your 18-stitch pattern.
the k2tog’s decrease 2 stitches into 1, so you go from 6 stitches to 3, but then you add those missing stitches back in by doing 3 of the yarnovers. the next 3 yarnovers prepare you for the last 3 k2togs, so you even out with 18 stitches in that set again.
if you haven’t started this pattern round yet, just count every 18 stitches, and place a marker. that will set you up nicely.
forgot to add… if you have any border stitches (the left and right sides), make sure you start your 18-stitch count AFTER those. so if you have 5 stitches for border, count 5, place a marker, THEN start the 18-stitch pattern counting.