Renaming a macro in C

This is what the concatenation operator ## is for:

#define XYZ(num) ABC_ ## num

Arguments to macros that use concatenation (and are used with the operator) are evaluated differently, however (they aren't evaluated before being used with ##, to allow name-pasting, only in the rescan pass), so if the number is stored in a second macro (or the result of any kind of expansion, rather than a plain literal) you'll need another layer of evaluation:

#define XYZ(num) XYZ_(num)
#define XYZ_(num) ABC_ ## num

In the comments you say that num should be a variable, not a constant. The preprocessor builds compile-time expressions, not dynamic ones, so a macro isn't really going to be very useful here.

If you really wanted XYZ to have a macro definition, you could use something like this:

#define XYZ(num) ((int[]){ \
    0, ABC_1, ABC_2, ABC_3, ABC_4, ABC_5, ABC_6, ABC_7, ABC_8, ABC_9 \
}[num])

Assuming ABC_{i} are defined as int values (at any rate they must all be the same type - this applies to any method of dynamically selecting one of them), this selects one with a dynamic num by building a temporary array and selecting from it.

This has no obvious advantages over a completely non-macro solution, though. (Even if you wanted to use macro metaprogramming to generate the list of names, you could still do that in a function or array definition.)

Yes, that's possible, using concatenation. For example:

#define FOO(x, y) BAR ##x(y)

#define BAR1(y) "hello " #y
#define BAR2(y) int y()
#define BAR3(y) return y

FOO(2, main)
{
    puts(FOO(1, world));
    FOO(3, 0);
}

This becomes:

int main()
{
    puts("hello " "world");
    return 0;
}