Multiply your tops and bottoms. That's the base for how many outfits you have! So, if you pack two tops and three pants, you have six days' worth of outfits because each shirt can be worn with each pair of pants. Dresses count as one day's outfit! I rotate between two sets of hiking outfits. And I only pack one baseball cap, one swimsuit, and one pair of sunglasses.  For underwear and bras, I take half the number of bras as my trip days and the exact number of underwear as my trip days. So if my trip is six days, I'd pack three bras and six pairs of underwear.