"It is impossible to involve such a high number of people into a conspiracy and keep them silent for such a long time."
This is the argument sometimes invoked by conspiracy denialists in an attempt to give a rational-sounding justification for their doctrine.
Unfortunately, it is fatally flawed in virtually every way:
The whole argument is based on the implicit premise that everyone who is in any way involved in a conspiracy is also aware of the conspiracy going on and willingly collaborating with it, which is clearly false; this premise completely ignores the hierarchical and highly specialized structure of our society, i.e.: the fact that virtually every aspect of our society is arranged in a pyramidal way, with a handful of individual rule over a big group of people (a minister over a whole institution; a CEO over its whole company, etc.) where the latter are just mere executors of orders coming from above and have no requirement to know what an order is really about in order to execute it; this guarantees that only a handful of conspirators are often more than sufficient to move huge masses of completely oblivious people towards a specific goal, if the conspirators belong to a very high place in the social hierarchy.
Even regardless of the aforementioned flaws, the argument does not actually prove what initially claims, i.e.: that conspirations are "impossible"; at best, it only proves is only that conspirations cannot be kept completely secret for a long time, but in no way provides justification as to why they are "impossible". In fact, not only it is perfectly possible for an elite to engage in a conspiration even at high odds of secrets being leaked to the public, but often they do not even need to keep them secret at all, since there is a vast array of very effective tactics to deal with those leaks making them harmless to the conspirators.
Last but not least, the theory behind the argument is flatly contradicted by practice, where conspirations of any kind are well known and recognized facts in both history and recent news (until they got arbitrarily removed from the category of "conspiracies"); rejecting conspirations means rejecting universally accepted evidence which, paradoxically, would imply a conspiracy in itself, making conspiracy denialism not only inconsistent with factual reality, but even logically self-contradicting.