When do we need to add 's' to a numeric year?
To answer based on what I think you meant from the clarification of the original:
1950 is specific, and 1950s is more general, referring to the decade 1950-1959. You would use 1950 only when you are referring to that specific year. So if you mean something within the decade 1950-1959, you'd use 1950s.
Am I answering the right question?
1950 refers to the single year 1950.
1950s refer to the period 195X, where X can be any digit.