The question above all questions: Who is Number One?

An answer is given indeed in THE PRISONER or, rather, a couple of answers, that is. Which does add even more to this particular puzzle. There is George Markstein who, along with McGoohan was the main source for the series, very much deserves this attribute. He is the bald man behind the desk, seen very briefly in the opening sequence, who is confronted with the apparent anger and presented the papers of resignation of the man who later will be known as Number Six. In reality things had happened vice-versa.

Information on George Markstein is scarce. Either, it is said, he was born in Berlin in 1929 and came to Great Britain in the 1930s when the Nazis took over power in Germany. Or else he was a US-citizen. He died early in 1987.
A journalist by profession, who had worked with the US Forces during the Cold War of the fifties he used to write political thrillers, worked for the British televison writing telescripts. He was script editor on THE ODESSA FILE, a movie directed by Ronald Neame.

He was aware of certain measures that would be taken during WW II in order to protect people who had delicate knowledge which could be valuable to an enemy. They would be sent "on holiday" so nobody would be able to get hold of them. A spot located very far away in Scotland called Invelair Lodge was one of those "holiday resorts" for high-profile officials. Markstein pursued this thread, going further as to keep the character of Number Six and the audience guessing which side was responsible for the abduction. This was also the subject of the novels he kept writing. As a consequence he regarded himself the inventor of THE PRISONER.

An English Wikipedia entry reports that Markstein had in fact developed a conclusion to the series. But because of his conflict with McGoohan he left the production and the idea was abandoned.
In his treatment Number Six, being a young agent, suggested measures how to get alongt with "retired" secret service persons, who would always be security risks, they would be kept in safety for the rest of their lives under unintrusive surveillance. Years later Numer Six discovered that his plan had been put into reality - but his idea had also been perverted. The Village he had conceived had become an interrogation and detainment camp. Number Six then staged his resignation from service and, as expected, was brought to that place. His intention was to learn as much as possible about it and eventually destroy it from within. But as a prisoner himself with many agents of different nationalities around, doubts arose as to which side ran the camp. Was it his service or that of "the other side"?

Eventually the entire world was a potential "holiday camp", he would have taken the Prisoner to the outside world where he, nonetheless, would have stayed a prisoner of his circumstances.

Both, McGoohan and he, had been working on DANGER MAN (SECRET AGENT), the PRISONER precursor. To Markstein it was obvious that Number Six was John Drake. McGoohan and he, opposite poles, are the fundamental currents at the basis of THE PRISONER which at the same time mark a distinguishing line: symbolic-allegorial, existentialistic the one; the other political-practial, down-to-earth in a way, straight forward oriented to the spy genre. Markstein left the production before the first 13 episodes had been completed. There was nothing left for McGoohan and him to discuss.

Markstein, in an interview, left no doubt on his attitude towards things developping in the series, "rubbish" the word he used. McGoohan, as he saw it, was an egomaniac who would proceed only his own way, control everything and would eventually drift into pointlessness. THE PRISONER, to him, was the work of a team of talented people like David Tomblin, Jack Shampan, the directors involved and of course the authors. It wasn't just McGoohan's "opera". On his leaving the crew he stated: "I walked out. That's why it became silly, you see. It needed a certain hand at the helm, you see."
And while McGoohan refused to comment on the series or the conclusion in particular, at the most he would give cryptic statements, Markstein did not like reading too much into it. And discussing a television series like THE PRISONER in university courses (like in Canada) was a rather absurd imaginaton to him.
Markstein in his own words talking for the 1984 documentary "Six Into One: The Prisoner File" (more... German language) can be read on Larry Hall's website (http://www.the-prisoner-6.freeserve.co.uk/markstein.htm; in English).

GEORGE MARKSTEIN INTERVIEWED
FOR THE 1984 SIX INTO ONE: THE PRISONER FILE DOCUMENTARY

To him McGoohan as Number Six, as a TV star he was a prisoner of his role, his face, his success. Regarding his own contribution he, with a good deal of self consciousness, was flattered by the thought being the real Number One.