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.
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.
Our SpeedFactor tool allows for monitoring and tracking all these metrics and values and more and reports on their evolution on a weekly basis.