Hello friends,
I would like to know the exact meaning of this idiom I have read on The Guardian today:

'Imperious Argentina gives Serbia six OF THE BEST'

Is there someone could help me?

Thanks, Jo.
Scored six of the best GOALS against them.
This is probably a title, thus things were simplified.
Additionally, "six of the best" means "six blows with a cane on the buttocks". It refers to corporal punishment in schools.

It's now used mostly jocularly.

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Is "six" a required word in that idiom?

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yes. It means they (metaphorically speaking) thrashed them!
Yes; six was the maximum number of strokes, in most schools – over here, at least. One or two strokes was more usual.

I believe there were also usually rules about how high the whacking party could raise the cane (usually, "not above the shoulder").