Normally, there are 2 formal, 2 semiformal and 3 casual per week. The exact number will be given to you when you get your tickets. For definitions see website
Hi, formal nights are eveninggowns long or shoet for the ladies and a tux or smart suit and tie for the men. Informal evenings are cocktail dresses or smart trousers and tops for the ladies and lounge suits for the men. Smart casual is smart leisure wear, not jeans and trainers though. Hope this help.