Is your question if you can have more than one function declaration in a header file? If that's your question the answer is yes, but there is no need to use multiple header guards.
1 2 3 4 5 6 7
#ifndef ADD_H
#define ADD_H
void add(int a, int b);
void add1(int a, int b);
void add2(int a, int b);
void add3(int a, int b);
#endif
Strictly speaking you don't need a header guard at all in this case.
ps.> if you read the above and study preprocessor directives carefully, you should understand that "ifndef" in your add.h does not necessarily need to meet the header file name. #ifndef FOO123_YEP wil serve just as good as far as some foo maniac have not defined FOO123_YEP somewhere else :)