These people are really idiots. If you want it chunky 15 - 17. if you want it smooth 20. (this is for Idaho or russet potatoes in particular) Cut your potatoes into appx. 3/4 inch cubes. Don't leave the skin on Idaho or russets or it will be like pieces of shoe leather in your potato salad. (I love potato skins otherwise). It might be fine to leave the skins on thin-skinned potatoes but I don't use those for potato salad.
Generally this depends on whether you want a smooth, mashed potato salad or a potato salad with a bit of 'bite' in it. If you want a smooth potato salad, then boil the potatoes for 30 to 40 minutes or so. If you want a chunky potato salad, then parboil the potatoes for about 10 to 15 minutes.