In canon, the Kaminoans are aware the Sith are in charge of the GAR cloning program. They are aware of the request for inhibitor chips in every single clone, and what these chips will be programmed to do. Enough of them know so that the Kaminoans deployed across the galaxy to support the GAR are aware and able to spot any risk of leaks to the plan on-site and attempt cover-ups ASAP.
In Legends, the clones themselves know what they are born to do, and are aware of the fate they themselves will hand to the Jedi at the end of the war.
Either way, that's millions of people who are aware of the Sith's identity, the existence and true purpose of the inhibitor chips and/or the true reason for the existence of Order 66.
Why is Palpatine so willing to let so many in on his plan? I know he may not have much of a choice if he wants to wipe out a whole Order of enemies, but even if he needs to let these people know, how is it that there doesn't seem to be any plans or contingencies to prevent or deal with potential leaks?