Format string vulnerability in C (how does stack behave in this case?)

Question

I came across an article about format string vulnerability in C and didn't understand how the format string %7$llx works.

Here is the code from that article:

#include <stdio.h>
#include <unistd.h>

int main() {
    int secret_num = 0x8badf00d;

    char name[64] = {0};
    read(0, name, 64);
    printf("Hello ");
    printf(name);
    printf("! You'll never get my secret!\n");
    return 0;
}

Command line input and output is this:

./fmt_string
%7$llx
Hello 8badf00d3ea43eef
! You'll never get my secret!

Article link here

So, I want to find out what happens in stack when printf is given that format string. As article states:

Due to how GCC decided to lay out the stack, secret_num is actually at a lower address on the stack than name, so we only have to go to the 7th "argument" in printf to leak the secret

What does the author mean when saying: "so we only have to go to the 7th argument in printf"? BTW, I understand memory layout and how stack works, it is just the printf and it's %7$llx format I don't understand in this particular piece of code.