There are 3 limits:
- a sheet may contain up to 4095 selectors, see
http://demos.telerik.com/testcases/4095issues.html - a sheet may @import up to 31 sheets, see http://demos.telerik.com/testcases/BrokenTheme.aspx
- @import nesting supports up to 4 levels deep
Microsoft support/MSDN reference links:
http://support.microsoft.com/kb/262161
http://blogs.msdn.com/b/ieinternals/archive/2011/05/14/internet-explorer-stylesheet-rule-selector-import-sheet-limit-maximum.aspx