Nobody knows. Not as many as what you read. More than we'd like.
The thing about depression is that there are different levels. Someone can be mildly depressed and still fit the criteria and some can be severely depressed. There is a wide range of how depression affects someone. Someone can have the blues and feel depressed but it is temporary and situational. Clinical depression is a much more pervasive problem into people's lives. There are statistics out there that would indicate it is pretty common and no one should ever feel alone if they are depressed. They certainly are in the company of others who are too. I wish the stigma that has surrounded mental health issues such as depression would be completely gone but still is not although it is better than past decades. The important thing is to ask for help if you suspect you are depressed.
Im sure a lot of people have it more then we know. I know I deal with anxiety and depression and its really hard. Also different people are effected by it differently. just because i feel certain things doesnt mean someone else will have the exact same symptoms.
iDK if there;s been any studies of people personally affected by depression. I do know the NIMH states suicide is the 3rd leading cause of fatalities of people between the ages of 13 and 30. I think that tells you its fairly common for young people to be suffering from depression.