One of the problems with that diagram is that the area from the Lincoln Memorial to the Washington Monument has the reflecting pool where the area from the Washington Monument to the Capital is all grass in the center area.
That said if you look at the aerial photos and other photos a lot of people are in the tree shaded area between the reflecting pool and the open grassy area.
How many square feet is the area occupied by people? If you allow 4X4 feet per person or (16 square feet)which will allow for some passageways and areas that aren’t full you can do a rough calculation of the number of people.
The pool itself is 2029 feet long by 167 feet wide. The original Mall was 400 feet wide with another 300 feet on either side of grass. Only 1 side was heavily populated + the center section. Add 400 feet on either end of the reflecting for balance If I did my math right that’s 1,501,457 square feet available or about 93k around the mall. If it’s 9 square feet per person it jumps to 166k. You’d have to add in the people behind Beck and on those on the other side of the mall by the lake. That crowd is pretty thick as well so you easily have 150k+