Look at some old photographs. They might not ALL have been on the beach, but it must have seemed like it. There were huge ques of soldiers waiting to get on the bigger ships. Surprising that even more weren’t killed minutes from rescue.
I’m sure once the civilian boats started getting there it was a massive rush, but Nolan doesn’t really show the beach at that point. Only time we get a good look of the beach in the movie is in the beginning when the goal was only 10% and most everybody was probably hiding in the buildings. I get the feeling that was “the people we might evacuate today” because you’re just not lining up everybody for a week, especially if you think you’re leaving 90% behind.