Menasseh ben Israel on the victims of the Khmelnitsky massacres of 1648-49.

In 1651 To His Highnesse the Lord Protector of the Common-wealth of England, Scotland, and Ireland the humble addresses of Menasseh ben Israel, a divine, and doctor of physick, in behalfe of the Jewish nation, the printed version of letters from Rabbi Menasseh ben Israel to Oliver Cromwell appeared :

There's a reference to the very recent Cossack massacres of 1648-49 (which actually lasted into the 1650s), known to Jews as the גזירות ת"ח ות"ט.

As you can see, his estimate of those killed were 180,000 with 600,000 surviving and still living in the area.

By contrast, the most famous account of the massacres, the יון מצולה by Rabbi Raphael Nathan Nata Hanover, himself a survivor, published in Venice in 1653 gives 235,000 as the number of dead. Other estimates of the time range as high as 600,000. It is difficult to know the true number. For more information about the number, see Shaul Stampfer's What Actually Happened to the Jews of Ukraine in 1648? in Jewish History 17:2 (2003), who gives a much lower estimate, and Jits van Straten's Did Shmu'el Ben Nathan and Nathan Hanover Exaggerate? Estimates of Jewish Casualties in the Ukraine During the Cossack Revolt in 1648 in Zutot 6 (2009), who tends to accept that a figure of at least 100,000 is appropriate unless evidence that the chroniclers exaggerated the numbers and that the later demographics can only work with a lower estimate emerges.