4 - 5 eyepieces are all that you really need. The first eyepiece I would go for would be a big widefield 20 - 30mm depending on the scope. these would normally be used to view extended DSO's that span larger than the moon like the andromeda galaxy which cover 6 full moons of the sky! their also used for Hunting down objects.
As for the second eyepiece, probably 15 - 18mm for a closer veiw of faint DSO's. The FOV of the eyepiece would vary depending on the scope and what you intend to look at.
3rd choice for me would be a 9 - 12mm for globular clusters, PN's and planets. As for a 4th choice, a 5 - 6mm eyepiece for really high magnification on planets and resolving the tight globulars and PN's. Keep in mind, these will only work when seeing is steady. If you get a Barlow lens, it will double your eyepiece collection by doubling the magnification on a given eyepiece. Remember it all depends on the scope and what you intend to view.
The formula for working out magnification is:
Telescope focal length/eyepiece focal length
Hope this helps
