When I was on the travel circuit I always did a carry-on and a backpack. The backpack was usually full of job related equipment so there wasn't much room for clothing etc. The carry-on was enough for trips of up to 2 weeks.

As I was in agricultural fields and hot temps everyday in all sorts of countries, full changes of clothing was required plus at least one laundry stop before I came home(phytosanitary rules and all). My carry-on often weighed in at 40lbs or more on those trips.