4
style.css is the generic one.
styleNN.css is for Netscape and Netscape-derived browsers, which usually loads the generic style.css and then changes any styles, which should be modified to look right in those browsers (not always necessary)
styleMAC.css is for - you guessed it - Mac's
I *think* that if no styleNN/MAC.css is found, it will load style.css
Edit: Yep, it does