How Common Is Sleepwalking?
Just how common is sleepwalking? The answer depends on who and where you ask. According to a 1999 study in the Journal of Clinical Psychiatry, a representative sample of about five thousand people from the United Kingdom revealed that only 2% of participants claimed to have suffered from sleepwalking. More recently, research done at Stanford University involving a sample size of nearly 20,000 people randomly selected from the US general population tells us that nearly 30% of participants reported having sleepwalked at one point during their life. 
Sleepwalking in the US
It is possible that the United States does have a much higher rate of sleepwalking, because sleepwalking results not only from genetic disposition, but can also be caused by sleep deprivation, sedatives, illnesses, and various medications. Since America is one of the most sleep-deprived nations on Earth, you would anticipate there to be a higher rate in American populations than abroad. 
Sleepwalking More Common with Depression
The same Stanford study found you are 3.5 times more likely to sleepwalk if you suffer from depression.  This means the commonality of depression is also linked with how common sleepwalking is. Studies have found that one in ten US adults report having depression. 
Adults vs. Children
Sleepwalking is more common in children than in adults. The studies on sleepwalking vary widely placing estimates anywhere between three to seventeen percent.  The most recent study from Stanford places the incident rate for adults at a new high of 3.6% up from the previously estimated >1%.  Boys are more likely to sleepwalk than girls are, and the occurrences usually peak between the ages of ten and twelve. Most children will outgrow the behavior once they enter puberty. 
The Big Picture
In summary, the estimate for sleepwalking's commonality in the public is between three to seventeen percent of the population, most active cases of which occur in children. The rate of sleepwalking will change depending on the rate of depression and sleep deprivation in a populace, as well as with the use of various drugs.