Last day I have been interviewed and the interviwer asked me as given the outline of a project, how can we determine the number of resources to be needed for the same?

I donot know to do do so?

Is there any standard way of doing so? or is it based on the experience? or how....

I am pretty new in this activity and my knowledge is zero at present .... so any clear explanation with some example(simple) will help me(and people like me) to understand this.

Thanks