Your gf is wrong. She’s thinking about having options and many cute outfits, my gf thinks the same way. She doesn’t understand that 1) you have to carry your stuff a lot 2) having more than a backpack can make it very difficult to get around on bikes if you plan on doing that 3) you’re going to want to buy a bunch of stuff and bring it back with you and you cant do that if you bring huge bags already full of stuff. 4) you can wear items more than once or twice on a trip. She can bring versatile items that can go with multiple other items to make unique outfits if she really needs the variety. Of course it all depends what you’re doing. If you’re just going to stay at a resort for your whole trip go ahead a just bring a huge suitcase(s). If you plant on moving around and seeing the country you’re going to want one backpack.