Probably not as many as older people who signed up for lots when we weren't sure which were going to be successful. Even then, my daughter has tumblr, myspace and even an ask.fm but they're not active. When you ask about the number of accounts people have - are you meaning in total, or actively maintained?
I have 6 in all and that is because I need to stay in contact with certain people, and prefer to do it that way.
I have 6 at the moment - Facebook, Instagram, Whatsapp, Viber, LinkedIn, and Twitter. Usually, it goes around this number, different people use different platforms, but mostly around 4 to 6 of them.