The answer is two players, and the official source you can quote is on Planeswalker Points: http://www.wizards.com/Magic/PlaneswalkerPoints/Information#LifetimeTotal
See the Lifetime Points section, first bullet on the “Here’s how it works” subsection, last sentence:
Casual events are always worth at least 1 point, and don't include participation points or event multipliers. A casual event can have as few as two players.
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