All patterns have a gauge to help you make the item the same size. It tells you how many stitches and rows fit into 10cm so you can make sure your knitting matches. If you have more stitches/rows then you need to loosen your tension (how tight you’re knitting) and if you have less stitches/rows then you need to tighten your tension (by pulling the yarn tighter when you create a stitch.
It’s always best to cast on more stitches than the gauge and knit more rows so that when you measure you can really see how many you have. Lay your sample flat and place your ruler on it to see where 10cm sits and count how many stitches/rows you have within that measurement.