Google launched Play Music back in 2011 with the option for users to upload their entire music collection for easy streaming. Play Music started with a limit of 20,000 songs, and it's stayed there ever since. Google has just announced a change, though. Play Music now allows up to 50,000 songs.

2015-02-25 12_48_17-Listen Now - Google Play Music

You should see a notice about the cap increase the next time you log into Play Music on the web. So, it seems to be active for everyone immediately. It doesn't look like there are any other changes to the Play Music usage limits—still only 10 devices and tracks can only be downloaded directly twice (unlimited through the music manager program). As always, everything you upload to Google is available for streaming and offline caching in the Play Music app.