Granted 1.5 to 2 million sounds very high, but since when is the DC Fire Department in the business of giving out demonstration size estimates - official or unofficial?

Estimating demonstration size in DC typically requires experience using aerial photos, taken over a period of time, as people typically come and go during a demonstration. Even then crowd estimates vary widely. Remember the Million Man March?

The estimates used to be provided by the Capitol Police, but they stopped after serious questions were raised by critics.

If the DC Fire Department wants to make estimates then they have an obligation to make their methodology and data public. Otherwise we can assume we’re just hearing an estimate from a political hack with an axe to grind - a result no more legitimate than those made by demonstrators themselves.