without wishing to be smart it depends on the size of the cup & the size of the oranges.Four oranges should give you a reasonable measure for a breakfast cup of orange. I find rather than the very large thick skinned ones the more medium sized are more juicy.
Microwaving your oranges probably isn't such a good idea as your immediately killing nutrients. you may find that adding 50% water to your juice is a good idea though. It tasts lighter and of course presents you with an immediate increase in volume.