When CD Projekt RED approached Sapkowski, they were a company who had absolutely no experience in making games. At that time there was already a one failed attempt of making Witcher game, and absolute catastrophe of TV series. No wonder he was sceptical of their success. Yet they produced an outstanding game: but still, they were small company, with no experience, no brand to speak of. Their officials claimed they need to sell one million copies of the game to just get even ("Musimy sprzedać milion egzemplarzy, żeby inwestycja w stworzenie gry się zwróciła"
source).
What they get was immense free publicity in Poland. Everyone was excited. People were talking about it. I was talking about it, despite not being a gamer (well, I did play games, but I was no more a gamer than someone who plays solitaire). I was talking about it to my foreign friends. There were articles in popular press. Simply put, CPR get a free marketing campaign in Poland, worth millions. The result: the game was I think the highest ever selling game in Poland ("W Polsce utrzymuje się na pierwszym miejscu najlepiej sprzedających się gier. Choć od premiery minęły zaledwie trzy tygodnie, już udało się pobić rekord wszech czasów: 67 tys. sprzedanych egzemplarzy. Najwyraźniej magia mistrza Sapkowskiego działa."
source). By 19.02.2008 the game sold 140.000 items in Poland, compared to 600.000 copies abroad (
source) - and the numbers abroad include items sold in Eastern Europe, when Sapkowski also was known and liked. They achieve one million sold copies by November 2008 (
source). However, they still had to get a loan to finance theirprojects (25 mln PLN
source). By 2013, they sold 5 millions copies total of Witcher 1 and Witcher 2 (of which 2.1 million copies of Witcher 1) (
source), 6 millions by the end of 2013 (
source), 8 millions by september 2014 (
source) and 25 millions by March 2017 (
source).
For comparison, by 2008 Sapkowski sold 1.3 million to 2 million copies in Poland alone (
source : "Jako ciekawostkę dodam, iż sprzedaż książek Sapkowskiego z Wiedźmińskiego cyklu, w samej Polsce, przekroczyła 2 mln egzemplarzy!", here 1,3 millions by 2005: "Od momentu debiutu autora do połowy 2005 roku książki Andrzeja Sapkowskiego rozeszły się w liczbie blisko 1,3 mln egz."
source.
The books were translated into Czech (1992), Russian (1996), Lithuania (1997), German (1998), Spanish(2003) when he even got an influential Award Ignotus Award in 2003, French (2003), Portuguese (2005). One unverified source claims he got 5 millions copies sold in Eastern Europe alone by 2005 (
not verified source)