Is a conspiracy of one a conspiracy? Yes. No.
I suspect the usual meaning of Conspiracy is "a bunch of people planned this and they haven't been caught yet"
Versus, LHO planned/conspired to kill JFK and he succeeded.
The usuage of the word Conspiracy usually indicates that people don't think the Truth has been made public and accepted as the Truth.
According to one of my team members who loves this Conspiracy stuff, JFK's brother Bobby went hard after the Mafia. Later, some CIA mission (cuba if I recall) tried to use teamwork to get the Mafia to do the job. They apparently botched it. So then, the Mafia had a chance to whack JFK in Dallas, so they did.
My details of somebody else's details are off, but I suspect the theory was formed to tie in Jack Ruby's actions to whack LHO (as he had Mafia ties and this cleaned up the mess).
In recent news, I did see an article on the JFK conspiracy topic. Lots of people make money off of it. And apparently, the Russians did indeed fabricate and circulate a document that tied LHO to the CIA, in order to stir up American distrust of their government. Oddly enough, the attempt itself failed, though some would say the distrust developed anyway.
Personally, I'm inclined to accept that a single gunman like LHO could indeed make the shot, but recognize that there were enough extra variables going on to cause folks to question that.
Additionally, as JFK's presidency declined in shame in the original timeline, things did work out better for him when the crew of the Red Dwarf set things to right and had him shoot himself.