Steam's catalog is still growing at a considerable clip. Tracking site SteamSpy said on Twitter today that 7,672 games were released on Valve's PC storefront over the course of 2017, while the 2017 summary page on SteamSpy itself says the actual number of new releases was 7,645. In either case, that represents a 53% year-over-year increase from 2016's total of 5,006 games released.
While 2017 was easily the busiest year of releases yet for Steam, the storefront's rate of growth is slowing somewhat. According to SteamSpy's annual summary pages, 2016 saw a 69% jump in new releases over 2015, which itself saw a 67% jump over the prior year.
One key part of 2017's growth of new releases was a major change to the storefront's curation system, as Valve ended the Steam Greenlight program (in which users voted on games to be allowed onto the store) and began Steam Direct, wherein developers could put virtually any game on the store if they paid a recoupable $100 publishing fee per game released.