It might depend on how cold and how warm/waterproof your outer layers are, I take 1 longjohn and 2 short sleeve, 2 longsleeve base layers and it is easy to rinse out in shampoo overnight, they dry quick. You could get away with 1 trews and 2 tops.
really depends on how active you are and on the temp on the slopes. last year i didn't need long johns, just used a base layer and some times a long sleeved t-shirt, bring a small rucksack and if your to warm you can take a few layers off. better looking at them than looking for them.
anyway nothing warms you like hot chocolate and brandy.