I ran my #ifdef counter on the Rotor sources.  The #ifdefs fall into a few categories:

1. cross-platform, cross-Operating System, cross-compiler: . Eg, _x86_, _ppc_;  PLATFORM_UNIX, Win32, _Win64, BIGENDIAN.
2. preprocesser alternatives to #pragma once.  Each of these would only occur once, at the top of a header file. These are the cases of :
      #ifndef MyFile_h
      #define MyFile_h
      ... contents of header ...
      #endif
3. Extra debug-only checks. (_DEBUG, LOGGING)
4. Toggling Features.

By far, most #defines were only used once (often as #pragma once alternatives).

I wonder how many other software projects have similar distributions.  I think it's interesting to consider how these could be reduced.
#1 (cross-platform/OS, compiler) could certainly be reduced by programming at a higher abstraction level. Using C/C++ as a shortcut for assembly language tends to produce a lot of #ifdef usage like this.
#2 (#pragma once) is purely a consequence of the C preprocessor's design choices. Other languages (like C#) don't even have this.
#4 (Features) is ideally solved by better componentization and better design abstractions.

Here were the top usages:

2249 _DEBUG
1266 DACCESS_COMPILE
389 _X86_
302 _MSC_VER
220 DECLARE_HELPERS
133 DEBUGGING_SUPPORTED
125 PROFILING_SUPPORTED
118 PLATFORM_UNIX
98 LOGGING
89 _PPC_
78 _DEBUG_IMPL
75 CHECK_APP_DOMAIN_LEAKS
74 __cplusplus
71 BIGENDIAN
59 _WIN64