It really depends on the type of project. However, my base point is generally reset, layout, pages. The reset fill is a reset, simple. The layout file establishes the primary layout of the site that is continuous throughout. The pages file includes styles that target specific pages based on a class that is assigned to the body for page identification.
Then I some times add on top individual module files that are responsible for styling a specific type of reusable component such as; data-grid, form, pagination bar, tab menu, etc. Any design component that is used multiple times in different contexts yet looks the same.
Generally as a project develops I tend to reorganize things as I gain a much better understanding of the whole picture. However, my base point hasn't really changed since I began developing professionally.
Once a project is complete the all the CSS is aggregated and served in a single request even though there several separate physical files. Most of the time… that gets a little tricky if you are dealing with open source projects, especially those with image references.