20k really is a lot for a board game, but I'm sure it sounds like "nothing" to someone who's not into the board gaming market. However, printing a game isn't cheap and you also have to think about that all these games needs to be stored somewhere, if not sold as printed-on-demand - which would of course make the price skyrocket (and you'd have to wait for some time in order to receive the finished product).
Most designers, especially those not backed by a huge game company or publisher such as Z-Man Games or Asmodee, only print a couple of thousand of copies when they publish their game, hoping to break even on that number or possibly earn a little! Way too many designers think that they're gonna earn money easy-peasy by creating a game and that considering how big the market is, selling "a couple of thousands" shouldn't be a big problem. Well most of them end up with piles of unsold copies in their garage or at some rented warehouse or container, until they're forced to sell them at a loss (at best).
Very few ppl actually manage to earn a living on thinking up and selling board games. Now ConcernedApe doesn't need to be concerned about this, naturally, since he already has conquered (yes, conquered) the digital market and have a great deal of money to invest in a product such as this. Still that doesn't mean that one can just produce an almost infinite number of copies. They still needs to be stored somewhere and you really wanna make sure that they do sell out eventually. Plus, there's always the risk of something going wrong in the process, be it something as easy as spelling errors or misprints of cards and whatnot. This is something that you'll want to fix in your next print-run, without having too many ppl end up with "faulty" products.
Printing 20k was a pretty bold yet well thought-out decision imo and I'm just as astonished by how quickly they all sold out, even if the Stardew Valley community is a big one - and so is the board gaming hobby. But because it is so big, regardless of having a famous IP/brand, you still have thousands of new and exciting games to go up against every year that hits the market through Kickstarter as well as publishers and game companies, big and small.
(would've understood it more had it been 3-5k)
