[Tails-testers] Testing Tails Test Releases

What does it mean to test Tails testing releases?

Is there a set of guidelines to follow? I haven't seen
any list of ways and/or methods of testing.

Are we to test it like we would a game and try everything
we could think of to find bugs?

How many Tails testers are there? I don't see much
activity in the testing ML so I assume it's a limited
set of people who routinely test or off and on, right?