Well, it's kinda like asking how many instances of a servlet are in a web app because there, too, there will seem to be only one instance.
Like a servlet, your portlet code must be multithreadable. That's why the action and render phases pass you the request and response objects, so when you have X number of users rendering your portlet, there will be X threads all invoking your portlet code with X requests and X responses.
doView() is an artifact of Liferay MVC framework built upon the standard portlet lifecycle so yes, it too will be called concurrently.
Sometimes developers get stuck on this, they'll use private member variables for local value stores between action and render calls and it will actually test out fine on their local system where they are the only ones invoking the portlet lifecycle. But when the code gets pushed to test and hit by many others, this code problem will reveal itself, sometimes in odd ways (depends, of course, on how it was coded).
