There is no limit to the number of levels of nested include files in C. However, your compiler might run out of stack space while trying to include an inordinately high number of files. This number varies according to your hardware configuration and possibly your compiler. In practice, it is not recommended to nest include files more than a few levels deep.

Nesting include files can make your code more difficult to understand and debug. It can also lead to circular dependencies, where a file includes another file, which includes the first file, and so on. This can cause the compiler to get into an infinite loop.

If you do need to nest include files, you should use them sparingly and carefully. You should also make sure that you are not creating any circular dependencies.

Some tips for avoiding problems with nested include files:

• Only include the files that you need.
• Avoid including the same file multiple times.
• Use relative paths instead of absolute paths when including files.
• Use a header file to declare the functions and variables that you need from other files.

By following these tips, you can avoid the problems that can be caused by nested include files.