The customer interview is an important aspect of product validation, exploration. Interviews are particularly important when you are trying to determine the next big thing that you want to introduce to your product or whether something is good or bad in your product.
But like almost all things in life, research and interviews in particular do suffer from the law of diminishing marginal returns.
Borrowed from the world of economics and business operational efficiency, the law of diminishing returns holds that an additional amount of additional insight will yield a diminishing amount of value to your insights driven mission.
The precise number depends on many factors, but suffice it to say that, if the number of questions remain pretty static and the pool of people you talk to remains somewhat homogeneous, it is likely that are five or six conversations, you will have reached a critical mass of insight that means that any additional conversations, corroborates what you already know but adds no new discoveries.
As an example. consider the question of whether a user prefers the choice of a set of radio buttons, check boxes or a list box to make a selection.
It probably becomes pretty clear that without changing the audience or the context of the problem to solve, that you will start to get the same answers predominantly, potentially even if you add more choices.
What you might see is more responses when you have free text options but those likely are encapsulated in terms and general understanding of design patterns.
