How many calculators do I need to have seen in order to make the claim that there are many calculators which are essentially 100% reliable?

Note that I am referring to actual physical calculators, not calculator apps on computers.

Well, to make the claim you actually made, which is that you haven't seen a single calculator that was wrong, anywhere from zero to all of them. It's just that the "zero" end of that spectrum doesn't really tell us anything about calculators.