How many nuts is a good number per day?

You only need a few. But if you like nuts, they can be a healthy replacement for chips. Just watch it if you are trying to lose weight.

I suggest you consider these factors:
What benefit are you looking for? Do you get that from any other part of your diet? What type of nut best provides this benefit, and how many do you need to eat? For example, almonds are rich in Vitamin E, but if you're have a spinach & broccoli salad for lunch, maybe you don't need a full serving of almonds.
Then you might want to look at these factors:
What is the impact of that volume of nuts? For example, calories. If you're eating 2000 calories per day, how many do you want to spend on nuts?
There is no hard and fast rule. Determining what works best for you will require some thought and planning.