How many rejected papers does it take to get an accepted paper? Today I received my first rejected paper. 

The committee of 2 people left some good points but at the end they didn't get the point of my theme. It took me 3 months to write this paper for the Requirements Engineering Conference in Lisbon, Portugal. 

I talked to 2 PhD students and they said that this takes time and passion. They told me that the average of rejected papers/journals per capita is 7. Do you agree with this average?
This varies depending on a huge number of factors.  The venues you select to submit your work, the quality of the work, whether the work is described well or penalized because it is written badly, misses important related work, or many other factors.  And it no doubt depends on field as well.  

I would however say that the idea that you submit 7 times for each time you get something published means you're setting your sights too high.  In Computer Science, for instance, probably most conferences have an acceptance ratio of 4:1 or 5:1, perhaps 6:1, at worst.  But then many others have much high acceptance ratios, so I would think the mean acceptance rate is 4:1 or 3:1.  In my own experience most papers have gotten in either the first time or the second.  One took 3, and another time I stopped trying to publish the work, but the average would be around 1.5-2 at most. 

Whatever the ratio is in your field, by all means don't get too stressed over your first rejection.  It's part of the process!  And work with your advisor to find appropriate targets that are realistic.