A Data Science Central Community
The sample size depends on so many things that even preparing a comprehensive list is a difficult task.
These are the stages of knowledge based on what you know / don’t know (assuming you have a clear understanding of what your dependent variable is):
a) Know almost nothing so you need to start by finding the independent variables defining in a satisfying manner the dependent variable;
b) Know the independent variables so you need to continue by searching for the function defining the relation between dependent and independent variables;
c) Know the independent variables and the function defining the relation between dependent and independent variables so you need to find the coefficients of this function.
What do you KNOW about the process under study?