How to show empty future posts, as a kind of placeholder

I have site which will have a number of fixed posts, for instance 20. I want to show a page with excerpts that have a fixed width and height, and when I have only four excerpts, I would like to have the rest of the page filled with empty grey boxes as to show where the rest will be placed in the future.

How do I do this? I placed a (for ($i=1;$i<=10;$i++) after the beginning of the loop, but that just gives me 20 times the same post.