Hi,I’ve been thinking of how can I develop a social network rest-api using Django,
my question is how many app should I need in my project?, I know that you will say it is depend on your functionality and requirements that you choose, but what I actually mean , when should I think of
functionality as a separate app and when to think about it as model or view in my main app ,
execuse me for my bad english because it is not my native languge 
I would suggest that you start with one main app, and develop the functionality you need in that one app. Once you have lots of functionality implemented, you might see that there’s a group of URLs, models, and views that could be broken off into a new app, but I don’t think you need to worry about that from the beginning.
This topic has been discussed a couple times here previously.
See:
as the two main threads on this topic. (This subject been touched on in a few other places as well, such as at Project structure and where to place new feature?, and Combining Urls, Views, Forms, and Models into fewer files)