how many placeholders is needed since i have nothing to show
You should have a Tribute page project you have already completed, so that would count as one. Maybe, have three extra placeholders, but for future design purposes, think about how your design would look/handle 20 projects. You will have many FCC projects to post on your Portfolio when you finish your certificates.
Most of us have nothing to show for that project. Just make up fake ones. I think I organized mine into rows and I had two rows. Don’t worry about it too much.