I’m of the view that this is a semantic question where we have a word, “pile”, that describes a general amount but doesn’t have a specified quantity to it, and so the only way we can determine the amount of units required to constitute a pile at the bare minimum, is through public consensus on the most commonly shared idea we generally have when we think of a pile.
I also think it’s possible for there to be a “range of graduation” between a non-pile and a pile, so for example “a non-pile becomes a pile somewhere between x grains and x grains” (depending on what most people think this range is), and if a given number of grains falls below this range, it would necessarily be only a minority of people that would still accept it to be a pile.
So I plan to count the answers here and see if we can come to some kind of consensus or at least most common or average opinion. For sake of not skewing the results, I won’t suggest my opinion on what I think the number or range of grains is upon which a non-pile becomes a pile. What do you think it is?
So, not really and answer to this specific question, but if you like this kind of problem I suggest looking up “emergent phenomena.” It’s basically about how scale can gradually but drastically change the behavior of a system. Going back to sand, for example. At an atomic level, sand is a solid, and just a couple of grains of sand behave very much like particles. But, when you get enough sand together, it starts behaving like a liquid.