maximum number of testers on google play alpha/beta testing

There is no effective limit (although there are membership invite limitations for Google Groups): Facebook, for example, uses alpha and beta channels for millions of users.

Here is a good link for reference https://support.google.com/googleplay/android-developer/answer/3131213?hl=en
Basically: Internal test: manage up to 100 testers, Closed test and Open test: there's no limitation