So here's something interesting. Due to the way achievement data was stored by Steam, people were able to reverse-engineer exact player numbers for those games that had implemented achievements, approximately 13,000 games or just over half the games on Steam. Details are provided in this Ars Technica article - https://arstechnica.com/gaming/2018/07/steam-data-leak-reveals-precise-player-count-for-thousands-of-games/
TSL is listed as having 1,529,038 players, which is pretty damn impressive given some of the titles with similar numbers. Of course the game is often on sale for as low as $2.50, so I imagine it has been an impulse purchase for many people. Although as pointed out in the comments, this data is only since achievements were added, so anyone that bought the game on Steam but never played it after the Aspyr patch came out is not included, so the true figure of all owners is probably closer to 2 mil.