Here’s a source on that claim. The uncertainty here is due to the large margin of error, so the takeaway is that it likely has no effect, or perhaps a small positive effect.

Here’s the claim:

the study estimates that for every 100 games that are downloaded illegally, players actually legally obtain 24 more games (including free games) than they would in a world in which piracy didn’t exist.

…

points out a number of caveats for this headline number, not least of which is a 45-percent error margin that makes the results less than statistically significant (i.e. indistinguishable from noise). That said, the same study finds that piracy has the more-expected negative effects on sales of films and books (and a neutral effect on music), singling out games as one area where piracy really does seem to work differently.