Just go to your compiler's include directory, and count the files, there can be dozens of them (Or hundreds. Or more.)
Java does not require header files like C/C++.
Platform dependent, usually between 10 and 10,000
C programs do not require header files. If you want a C program without header files, you can simply not create them. However, you may or may not be able to include your non-header file source files.
list of header files in c and function prototype associated with each file
It isn't necessary to include header files in C. However, without the functionality provided by some header files, your program wouldn't be able to do very much that is useful.
256
Yes.
No predefined 'header' function in the standard C libraries. There are header files, if that's what you mean.
The C header files are in the same place as other Unix and Unix-like systems: /usr/include if you installed the compiler.
No.
The source file must include the header file. Beyond that we can only guess at the problem without seeing the content of the source and header files. Do not post the files here. Such questions are better handled by the many C programming forums available elsewhere on the Internet.
The need to declare header files is not compulsory in C++. You may place all your code in a single source file if you so desire. However, header files are useful in that they separate interface from implementation and aid in hiding information.