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One last donkey, partially visible, in the lower left.
Final answer: 8 donkeys!
If you only saw 5 or 6 at first glance, don’t worry: that’s precisely the point of these kinds of challenges. Illusions and visual tests push the normal limits of our perception and attention.
And you, how many did you find before seeing the solution?
Tag a friend who thinks they’re super observant and challenge them! 
Note: The number “8” corresponds to the number of donkeys seen in the image; I cannot confirm that this is the number the creator of the image intended.