Yes, too many recursive calls may result into stack overflow. because when a function is called its return address is stored in stack.

After sometime the stack memory will be filled completely. Hence stack overflow error will occur.

When a recursive call is made, the function/process clones itself and then process that funtion. This leads to time and space constrains.

In a loop, there is no recursive call involved that saves a lot of time and space too.

Yes, It will return the value 20*20 = 400

Example:

#include <stdio.h>
int f1(int, int); /* Function prototype */
int f2(int); /* Function prototype */

int main()
{
    int a = 2, b = 3, c;
    c = f1(a, b);
    printf("c = %d\n", c);
    return 0;
}

int f1(int a, int b)
{
    return ( f2(20) );
}

int f2(int a)
{
    return (a * a);
}

Output:
c = 400

No, C can accept upto 127 maximum number of arguments in a function.

Yes. If a function contains two return statements successively, the compiler will generate "Unreachable code" warnings.

Example:

#include<stdio.h>
int mul(int, int); /* Function prototype */

int main()
{
    int a = 4, b = 3, c;
    c = mul(a, b);
    printf("c = %d\n", c);
    return 0;
}
int mul(int a, int b)
{
   return (a * b);
   return (a - b); /* Warning: Unreachable code */
}

Output:
c = 12