TL;DR below

The Sun - June 6, 1915 - fifth section - By Herman Bernstein, Editor of "The Day" : link

Horrors Worse Than Kishineff Charged Against Russia To-day

Unparalleled Conspiracy to Crush the Jews Alleged to Be Organized to Cover Up Defeats of Czar's Troops - - - Torture and Massacre Declared to Be Rife in Hundreds of Towns

Since the destruction of the Temple in Jerusalem the Jewish people have had no darker page in their history than that which the Russian Government is writing to-day. Six million Jews, one-half of the Jewish people throughout the world, are being persecuted, hounded, humiliated, tortured, starved. Thousands of them have been slaughtered. Hundreds of thousands of Jews, old men, women and children, are being driven mercilessly from town to town - driven by the Government, attacked by the troops of their own country, plundered and outraged.

In another part:

Dear brethren, have mercy on the six million Jews in Russia and take our part! Ask the Russian Ministers why we are being tortured so mercilessly. Our children are slaughtered in the Russian army.

New York Times - October 18th, 1918 - page 12 - : link

$1,000,000,000 FUND TO REBUILD JEWRY

Six Million Souls Will Need Help to Resume Normal Life When War Is Ended.

LOANS WITHOUT INTEREST

Committee of American Jews Lays Plans for the Greatest Humanitarian Task in History.

The American people, Jews and non-Jews alike, will soon be asked to lend or contribute the larger part of a fund of approximately $1,000,000,000 to carry out plans for the reconstruction of the Jewry of the entire world. Announcement to this effect, together with the general plan of procedure, was made public last night, following a meeting held yesterday in the office of Felix M. Warburg of Kuhn, Loeb & Co., Chairman of the Joint Distribution Committee of the American Funds for Jewish War Sufferers.

While the appeal will go out to the entire world, the people of war-ravaged Europe, it is believed, will not be in position to further the plan financially to any large extent, so that burden will fall upon the United States, Canada, and perhaps England

The money for the project, the largest purely humanitarian undertaking in history, will not be sought alone through contributions, but will embrace loans, and will be accepted from non-Jewish as well as Jewish sources. The exact date and duration of the campaign for the billion-dollar reconstruction fund was not announced...

and later

6,000,000 Jews Need Help

New York Times - September 8th 1919 - page 6 - link

Ukranian Jews Aim to Stop Pogroms

Mass Meeting Hears that 127.000 Jews Have Been Killed and 6.000.000 Are in Peril.

We come out now before the world with the determined slogan ‘Those pogroms must stop’,” said the President in his annual message. “It is only a question of holding these facts continually before the civilized world; we must not permit the world to slumber. This fact that the population of 6.000.000 souls in Ukrainia and in Poland have received notice through action and by word that they are going to be completely exterminated.

TL;DR

The successive blows of contending armies have all but broken the back of European Jewry”, he said, “and have reduced to tragically unbelievable poverty, starvation and disease about 6.000.000 souls, or half the Jewish population of the Earth.

HALF THE JEWISH POPULATION ON EARTH IN EUROPE

Dear bretheren, have mercy on the six million Jews in Russia and take our part!

The other half?

It is only a question of holding these facts continually before the civilized world; we must not permit the world to slumber. This fact that the population of 6.000.000 souls in Ukrainia and in Poland have received notice through action and by word that they are going to be completely exterminated.

Wait, what? How'd we get another 6 million?

More pre-WW2 mentions not listed above, sources available upon request:

Six Million Jews Face Starvation... ...Bad Conditions in South-Eastern Europe Reported by Rabbi Wise. Fears Crisis at Hand.

So it's Six Million in South-Eastern Europe?

A depressing picture of 6.000.000 Jews in Central Europe deprived of protection or economic opportunities, slowly dying of starvation, all hope gone

So now it's Six Million in Central Europe?

Let's list this out:

-6m in EUROPE

-6m in Russia

-6m in Ukrania/Poland

-6m in South-Eastern Europe

-6m in Central Europe

6 million, you say?

Video source of actual newspapers