netadictos makes some good points and I would concur. It's easy to seek reasons for more Css but the benefits of keeping them lean are far greater in the longer term.
In addition, have you looked at using themes and skin files within asp.net? The combination of .css and .skin can dramatically reduce the overall size of your Css, which is marginally good for performance but very good for easier administration.
Some exceptions could be contained within the one css file but if things are radically different within the one then you may consider a separate css or even a separate site if they are that different. Obviously you might load different versions of the themes depending on which user it is. This is where you could have an explosion of Css files. That is, say you had a css for each page and then you wanted to have different for different clients on your site, then you'd be growing exponentially. This of course assumes you have this challenge.
