The amount of documentation available for any one ancestor would not plot out on a Bell Curve. It is a historical fact of life that documentation diminishes as you backward in time. At some point in the past, finding even one source document for an individual would be a major accomplishment. But how many sources are really sufficient to establish the historicity of your ancestors with a reasonable degree of certitude?
Of course, this whole issue is completely lost on the vast masses of casual genealogists who eschew sources and have none at all for any of the people in their pedigree.
Part of the answer to the question depends heavily on the temperament of the researcher. Researchers with a strong or even obsessive collection instinct will continue to accumulate source indefinitely. There is never "enough" until the last scrap of paper and torn image have been neatly tucked away in the never ending database. Fundamental to the challenge of accumulating evidence is the need to have multiple sources. Every record you acquire either adds more information of the ancestor and the family or provides clues for the extension of the line to another generation. But eventually, gathering more information begins to impinge on the category of biographic rather than genealogical.
The risk of failing to consider any possible additional sources is that you will fail to find that one crucial piece of evidence explaining all of the little and big mysteries that accumulate in any research effort. In my own family, there is a tradition that my paternal grandparents had a "stillborn" daughter. The information came from my father's personal accounts. But as of yet, we have found nothing to confirm that fact. But there are still dozens, perhaps hundreds of letters and documents out there waiting to be found. Any one of which, could provide the missing information. Do you keep looking and keep accumulating when there are real issues to be resolved? In my case, it is entirely possible that I already have the information "somewhere in my files" answering this and many other questions since I have not taken the time to read every letter and examine every document in all of the boxes and boxes stored around the house.
But starting out with some basic concepts, I think that there is a definitive answer to the question of the amount of evidence necessary to prove any particular assertion of fact. But rather than being a strictly quantitative question, the amount of proof sufficient to prove a fact turns out to be highly dependent on circumstances. It was not uncommon when I was preparing for a trial while practicing law, to get into a lengthy discussion with my clients about wether or not a particular document was a necessary piece of evidence in a case. Most clients had the tendency to want to "tell the story" even when telling the story had nothing to do with proving their case. They also constantly felt that if one or two documents were necessary, then fifty or a hundred would be better. The answer was that once a fact had been established, the burden of proof passed to the opposing party and unless additional controverting evidence was found, then the fact asserted was established. In fact, there is a procedural rule that will prevent repetitious and unnecessary evidence from being presented during a trial. It is not infrequent to have objections if a party tries to repeat proof more than one time.
But does this legal principle apply to genealogy? Yes and no. It is true that confirming evidence is not indispensable, but it is helpful. There is no genealogical judge sitting there granting your objections and those of opposing counsel, so gathering additional supporting evidence is usually a good idea and could be helpful. Unlike a court case, as genealogists, we are really out there to "tell the story." Anything we find that assists in adding details to the story is pertinent and should be preserved. If the amount of information accumulated is so great then the collection may be a candidate for inclusion in an established library.
At this point you may be asking, well answer the question, how may sources do you need? The simple answer is everything that you can find that has any information about the ancestor. If you are asking the question with the hope that I or anyone else will tell you to stop looking then you are in the wrong pursuit. In genealogy, there is never, never enough evidence and there are never, never enough sources.
Now at this point, someone will raise the Genealogical Proof Standard. But I am not talking about the need to prove any one fact, I am talking about preserving the details of the lives of our ancestors. Details that will inevitably lost without the effort of preservation. It may be reasonable to stop trying the same issue over and over again when it is no longer in dispute, but the additional documentation may be directed at telling the story and not proving the point.