Is "sky" a countable noun or an uncountable noun?
We can count the sky as it is only one, but it's that people refer to as it being uncountable.
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If we use the term our usual way, yes, there is one sky.
Cambridge Dictionary, however, also has this definition:
skies [ plural ]
the sky in a particular state or place: For weeks we had cloudless blue skies. We're off to the sunny skies of Florida.
Ngram shows also significant use of 'skies'.
We may deduce that it is countable.