There is only a limit of 100 testers in internal testing. Other channels (Closed testing, open testing and production) doesn’t have any limit at all, but every update of app needs Google approval, which can take some time. Also, Google needs to approve post of the app (title, description, icons, etc…) even without uploading an app which can also take some time. Source: I’m developer (not of this app of course)
There is only a limit of 100 testers in internal testing. Other channels (Closed testing, open testing and production) doesn’t have any limit at all, but every update of app needs Google approval, which can take some time. Also, Google needs to approve post of the app (title, description, icons, etc…) even without uploading an app which can also take some time. Source: I’m developer (not of this app of course)