Note: This writeup pertains to the game of Nethack.
A wish is a handy opportunity resulting from zapping a wand of wishing or rubbing a magic lamp. You can occasionally get a wish by quaffing a smoky potion, sitting on a throne, or (very rarely) drinking from a fountain. It isn't a true, abstract wish, because all you can do is ask for an object, but it is still quite a boon.
Unless you feel very lucky, wish for no more than 2 of the item you want -- there is a 60% chance of actually receiving two; otherwise, you just get one. Chances of getting 3, 4, or more of an item (except zorkmids) rapidly approach zero.
Similar logic holds for enchantments greater than +3.
What to wish for?
If you are using a wand of wishing, the first thing you wish for should be 2 blessed scrolls of charging. Otherwise, popular items to wish for include:
Needless to say, wishes are highly useful in
saving yourself from imminent death. Under those circumstances, wish for a
unicorn horn, a
potion of full healing, a
scroll of genocide, a
scroll of fire, a
ring of slow digestion, whatever it is you
need, because all those stockpiled wishes don't do you any good if you're
dead.