You have 7.
Your “Sold” sign will be for the villager No. 8.
Asking your camper, Eunice, to move in…she would be your No. 9.
Since you do not have a plot for her…she cannot move in yet. She will tell you Tom Nook informed her there is not enough room. This means…go ahead and place that next plot so Eunice can be No. 9.
This is actually smart because you get to control, at the beginning (when not all ten slots have been filled), bringing in villagers slotted between Nos. 7 to 10.
What you will not be able to do is go to Mystery Island and expect to find villagers. They appear there if you have an available plot. (A vacancy.)
Also note: The villagers appearing on your map [screen] are listed according to seniority; meaning, the game date on which they moved to your island. (Each one’s first day.) So, no two villagers will move in on the same game date. And your No. 10 will not try to move from your island.