Reads every site this instance hosts and walks them one at a time, then says how many there were. The shape to copy when you want to do something to each of a list — inside the loop, {{item}} is the current one and {{index}} is its position.
The steps
Every site you host, listed runs sites → each_site → name_site → count_sites.
Who can run it
Every site you host, listed runs as-is on the public site once it is saved to your account.