There are various points you need to think about when considering how many stair tread lights you need but usually, it would be about a step light for every third step but, he height of your steps or stairs will affect how many step lights you need.
Also, think about the brightness of the units you either have or are buying as they do vary. The light temperature will also make a difference, color temperature options can vary from bright white or daylight to a soft warm glow.