The Google Play Developer Console has changed a lot since the initial response.
If you are using Open Beta Testing, follow the link:
Release Management / Application Release / Beta Management
and you will see something like this:

Note. If the number of testers is not displayed, this may be due to the fact that the number has not yet reached a certain threshold.
This specifically answers the question "How many people chose beta testing?" which may or may not be what the OP really interests.
Analytics will be a way to find out how many of them are actually using the beta.
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