Classical Greek Philosophers -- How Widely Read?

In Plato’s day, how many copies of his collected dialogues would have been written down? Who would have had them? How widely would they have been distributed?

If Plato died in 347 B.C. in Athens, how much of his lore, at that time, might another philosopher in, say, Corinth, have been familiar with? Were there any kind of correspondence societies, where a dinner-and-philosophy club in Thessalonika would all be happy because they just got a copy of Plato’s most recent publication?

What was (in rough terms) the speed of transmission of the great writings, Plato, Aristotle, and their ilk?

(How “dark” were these ages?)

How familiar would Plato have been with, say Democritus (died 370 B.C.) or Heraclitus (died 470 B.C. – an additional century to be known…or forgotten?) I’m not really asking for these specifics, but just a kind of generality. Were the works of the great philosophers spread around for others to know, or was everyone pretty much on his own, in whatever small school or academy he might start up?

(Inspired in part by the thread of epicureans vs. stoics.)

If you mean reading in the modern sense, via the original texts, not that widely or fast at all due to no printing press, lower literacy rates, and slower pace of transportation, making distribution limited—but other cultural methods made up for it and make the comparison with modern readership inapt. From Macaulay’s essay on Samual Johnson:

Fun quote! And great insight! My guy in Corinth might never actually read a word of Plato…but someone from Athens, passing through, might repeat at least a summary of all that’s hot back home, making the ideas, if not the exact words, available. Thank you!

I doubt we have an accurate numerical figure for the number of manuscripts for any ancient book, but we can be sure that the number was very small. Parchment and ink were expensive, but more significantly, it took a large amount of time to produce even one copy of a full-length book. Scribes were costly, being among the most skilled tradesmen, and their time was valuable, hence the cost of producing any book was high.