That's a tough question. It depends a little if you are looking at individuals or groups. If you are looking at groups you could do an observation of a school setting at the same place at the same time every day for 30 days. Say a college cafeteria or frequent hang out place on campus. How many selfies are taken every day in a college cafeteria for example.
If you want to know how many selfies individuals take every day, you'd need an observer with them to document it - college roommate maybe.
A big issue with observational studies is the observers influence on them. When people know they are being observed, their behavior changes at least initially. There's a tendency for the behavior to go back to 'normal' after a while.
I observed critical care nurses at work. Initially they were more concerned about what they were doing and saying. After a while I became more like wallpaper.
Interesting question, I would try an empirical research using mine or other people’s (i.e. friends), social medias. Use your time/date and look for selfies posted within that time/date. If you start with your friends, then your friend’s friends so-on you should get a large empirical study group. Statistic out each social media: Facebook, snapchat, and Instagram then total all, so-on. Luck.