I've done 2 years here, haven't been to loads of balls, just a few. My advice, if packing is an issue, would be to leave out the ball gowns - they're not essential. In fact, as far as I could see, long dresses arw more commonly worn. Of course, if you want to wear a ball gown (like me, as I own one) go ahead. I think that if you own cocktail dresses, there are occasions that suit those too, so pack one (I say one, this is because I am poor
so only own two dresses in total). For the May Ball, I think a really nice just-below-the-knee dress would be ideal as a compromise between formal and getting wrecked in the mud, but of course adapt as you wish. None of the things I went to had really specific hard-and-fast rules.
For the worried person, here as everywhere there are people who think outfits - and lots of them - are really important, but there are also many who don't care much, so you can take your pick. I have minimum clothes and I've been just fine. If you don't have money (or inclination to shop), I don't think your friends will mind you wearing the same dress to several balls! And many people go to only one or two a year, or none at all. Just depends what you want.