In 1905, the Jewish population of the Russian Empire (including Poland) was counted as 6,060,415. This figure was still being used by American Jewish sources in 1917 (AJC Yearbook, 1916-1917, p. 276). It therefore logically follows that American Jewish organizations seeking aid for their brethren in that region would cite a 6 million figure.
However, the Google News Archive returns these hits for the period 1905-1939:
"Five Million Jews": 42
"Six Million Jews": 44
"Three Million Jews": 57
"Millions of Jews": 1,520
Moreover, stories relating to the imminent starvation, deportation or annihilation of 5 million (not six) can be found here and here and here and here. Similar stories referring to "three million Jews" are here and here. The second article asks for help "so that three million Jews shall not perish from the face of the earth."