Apple TestFlight iOS 8 - What is the limit for external beta testers?
As of November 13. 2015. there are some changes occurred in iTunesConnect:


  To make your TestFlight experience even better, we’ve made some
  significant changes. You can now invite up to 2000 external testers
  and up to 25 internal testers per app. You can also test up to 100
  apps at the same time for up to 60 days, and submit up to 6 updates
  per day to your external testers.


So, right now we can invite 2000 external testers per app.