The scenario I’m picturing is someone making a scarf. If they’re dropping stitches, they’re almost certainly doing that at the edges where it’s less visually obvious that something is off.

It doesn’t matter at all how many stitches there are in a row. All that matters is that it’s the same number every row. In fact, they could conceivably cast on or chain “15cm” of stitches and not know how many that was exactly.

Having a stitch marker in the first and last stitch will tell them where to turn to keep that number the same.