What is proper size of buffer when using sprint()?

What is proper size of an char array (buffer) when i want to use sprintf function?

I dont know why this part of code is working if buffer can hold only 1 char? I put a lot more chars inside than 1.

/* sprintf example */
#include <stdio.h>

    int main ()
    {
      char buffer[1];
      int n, a=5, b=3;
      n=sprintf (buffer, "%d plus %d is %d", a, b, a+b);
      printf ("[%s] is a string %d chars long\n",buffer,n);
      return 0;
    }

Results:

[5 plus 3 is 8] is a string 13 chars long