Facts
  The reflecting pool on the National Mall covers an area of about 8 acres. [reference]
  An American football field including the endzones covers about 1.3 acres. [reference]
  The seating area of Michigan Stadium covers about 6 or 7 acres. [reference]
  The capacity of Michigan Stadium is about 100,000 people. [reference]

Observations
  At the most crowded locations, the density of people on the Capitol lawn today was no more than in a packed stadium. [references: lawn, stadium]
  Most of the people on the mall today were within an area of two or three reflecting pools in size (and white). [reference]

Estimate
  (2.5 reflecting pools) × (8 acres per reflecting pool) ÷ (6.5 acres per stadium) × (100,000 people per stadium) × (average 0.5 density) = 150,000 people. More than 87,000, perhaps. Hundreds of thousands, as in more than 200,000? Not too likely. A million? No.