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A maximum of four.
And I would still argue that is too many already. Considering the need for frameworks or site-wide libraries, four is the maximum I recommend.
How Many CSS Files Should a Website Have?
A maximum of three.
Although I went for two for a recent SaaS, where one CSS file was a library (grids, UI, fonts) and the other one had custom styles.
Also, see what Chris Coyer thinks about this.
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