I am a one bike person, with the caveat that I keep a bike here, and have one in America for when I am home for tours and visiting. The bike here is a BMW f650 and the bike there is a Harley Davidson. Long story on why I have the Harley but not relevant here.
I consider the 650 single to be the optimum one-up adventure touring ride. They are comparatively cheap, but have good power and snap for one person. They are lighter and simpler, repairs are eary. They are easy on gas and insurance is cheaper. THey can take a fair amount of load (gear) but they still handle any legal highway speed basically anywhere on earth, which is better than bikes with even smaller enginers. Whichever brand you like is a personal choice, but for true adventure touring alone I would be hesitant to go larger.
If in future we tour two up with my wife, I will go to something larger. Until then I don't see compelling reasons to do so.