The vast majority of touring bikes have triples... three chainrings. I wish I could find the good article I read which critiques the average triple. In short, it said many triples have three lousy chainrings: the lowest one doesn't go low enough, the middle one is unneeded because it offers nothing new, and the highest goes too high.
Ideally, touring bikes would come with just a single chainring. Shifting would be much simpler... just move one lever one step to get to the next gear. But we're not there quite yet... we'll need an affordable 10-50 cassette to give a good range (18-100 gear inches, for you bike-math geeks). And frankly, we may never get there for touring bikes... you'd need at least a 12-speed cassette so that your gears aren't too far apart, but a 9- or 10-speed is the sweet spot for touring today.
A double is the perfect compromise; think of it as a single, but with an extra chainring for uphills. For flat land, down hills, and even slight uphills, simply stay in the bigger chainring. For uphills, including hills that level off for a bit and then continue up, stay in the smaller chainring.
The advantages of a double over a triple are:
It's simpler and less intimidating for a new biker:
No more hunting for the middle chainring; with a double, the shift lever is either up or down.
No need to recall which way to push the shift lever when leaving the middle chainring (up, or down?)... with a double, no thinking needed!
All common gears are accessible from the big chainring... no more switching between the middle and upper chainrings.
It's more biker-friendly because a double is more narrow than a triple, which results in:
Somewhat snappier shifting.
Decreased drive-chain wear since the chain is not angled as much, especially with a 9-speed.
Lower Q-factor, especially important if you don't have wide hips. The Q-factor is named from the word "quack", the sound a duck makes. You'll look like a duck waddling if the Q-factor is too large for you. OK, that's an exaggeration, but you get the point, and millimeters make a difference for knee health and efficiency when pedaling all day.
These are not huge things, but small things that add up. The main thing you give up by not having a triple is a little range... approximately the loss of the highest gear, which is rarely used anyways.
I think a good gearing scheme is a 22/40t double crank with a 9-speed 11-32t cassette (assuming 700c wheels; if 26 inch, use a 24/42t instead). It's recommended that you pedal from 60 to 90 rpm (revolutions per minute), so...
You'll generally be in your big chainring traveling 9 - 26 mph (at 90 rpm).
Up hills, you'll be in your small chainring traveling as low as 3.3 mph (at 60 rpm, 5 mph at 90 rpm).
This gives you thirteen unique gears with five gears in the common 10 - 20 mph range. Note that in the small chainring not all gears may be usable, but the higher gears are redundant, anyways.
You'll find Sheldon Brown's Gear Calculator indispensable when you're thinking about gearing.
Unfortunately, wide range cranks like 22/40t are not available off-the-shelf. Fortunately, they can be built up using parts from Dimension, ordered complete from White Industries, or created by substituting a chainring on an existing double crankset. If you know me, talk to me before going this route for more info.