Yeast nutrient, as the name implies, contains nutrients the yeast needs to function properly. Wort is very nutritious, so adding nutrient is rarely needed. If you are making a cider or wine, sure, but for beer, not really.
As far as pitch rate goes... adding nutrient doesn't really change pitch rate as far as I know. The yeast will need to reproduce until reaching a certain size colony, dependent on volume and gravity, and how much nutrient you provide has no bearing on the ideal colony size. Granted, a bigger colony needs more nutrition, but that should already be in the wort. The only nutrients that are not available in abundance in a typical wort are zinc and oxygen. If you aerate well, zinc is the only nutrient typically in short supply in wort (well, O2 still is in short supply with good aeration I guess because at the homebrewing scale we can only do so much to get enough O2 into the wort). A small zinc deficiency is not really going to make or break most fermentations.
I would pitch according to traditional pitch rates, whether I was adding nutrient or not. It's no different than using a Wyeast smack pack, which includes nutrient - you follow standard pitch rate guidelines.