How many people can fit into the pentagon, which was once the largest office building in the world? assume that everybody must be standing on the floor?

The Pentagon has a floor area of about 600,000 m^2. If you put 2 people in each square meter, that about 1.2*10^6 people. The nearest people is 10 is 6. Of course, the logistic of moving over a million people into and out of the Pentagon would be nightmarish.