Three people in a conspiracy is plausible.
Ten people starts to become a little unbelievable, particularly for as outrageous a proposition as assassinating a President. Would someone know nine other people that one can safely approach to invite them to join this scheme? Even if one thinks one knows nine others, is it not the possible that one of them might decide to become a big hero. If one of them alerts the authorities, and the gunman is captured within the last hour red handed with the rifle, the person who turned them in would be a big American hero. Who could resist such a prize? If there is only a ten per cent chance that a certain person would betray the conspiracy in hopes of becoming a big American hero, the odds of success are just under 35 per cent, with about a 65 per cent chance that up to nine would go to prison.
By your logic, no conspiracy would ever succeed.
Here in the real world, criminal conspiracies happen and often succeed. In the criminal underworld, where violence is a commonly used tool, it's easier to get people to keep secrets.
Aside from organized crime, the CIA also uses tools like deception and plausible deniability to execute conspiracies around the world and in some cases they've collaborated with the criminal underworld.
Of course, the CIA isn't the only intelligence agency that kills people and uses deception and plausible deniability. There are foreign intelligence agencies that use similar tactics. So don't think I'm only pointing fingers at our American spies.
Lastly, organized crime and intelligence agencies use "compartmentalization". Which means that people could unwittingly be part of a conspiracy and not even know who they're working with or what the over-arching objective is. Sort of like how it's alleged that some of the 9/11 hijackers didn't know they were on a suicide mission. They were only told what they needed to know to complete certain tasks. They may not have been told that it was a suicide mission.
https://www.theguardian.com/world/2001/oct/14/terrorism.september111So, I have answered your question. How about answering mine. The side without the truth on their side will always dodge questions. Both questions address what you believe probably happened, not what you know for certain happened.
Question 1:
What tasks did the conspiracy successfully complete (the assassination, faking the Zapruder film, faking the autopsy reports, etc.)?
Question 2:
How many people would you estimate would be needed to do all that?
First off, I don't believe the Zapruder film is "fake".
Secondly, the problems with the autopsy are part of the Cover-Up, which I view as separate from the Conspiracy plot to kill Kennedy.
The people involved with the Cover-Up likely had different motives than those who plotted to kill JFK. For example:
- LBJ suspected others were involved but wanted to prevent WW3
- Some wanted to cover up incompetence by law enforcement and the national security agencies
- Some had an interest in protecting secrets related to operations against Cuba or the Soviets (ie RFK)
- Some had an interest in protecting secrets related to the CIA's domestic spy programs (it's illegal for the CIA to conduct operations domestically)
Therefore I view the assassination and post-assassination cover-ups as separate (but related) issues. Covering up evidence of a conspiracy makes them indirectly complicit after the fact.
Also, some agencies (ie the FBI and CIA) have admitted to covering up information about the Kennedy assassination so it's not even debatable that there were in fact cover-ups and that leads to valid questions about the evidence.
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To answer your questions, I believe a minimum of ten people would be needed in order to execute a plot against JFK. Two to three people handled Oswald while the rest handled the logistics of what happened in Dealey Plaza.
Whether that qualifies as "large conspiracy" likely varies from person to person. I personally don't think ten people is a "large enduring conspiracy".