When did the number of Jews decrease?

Answer 1

This happened several times in history. However, the most recent and largest decrease in the number of Jews occurred during World War II, when the Holocaust claimed the lives of 6 million European Jews.

Answer 2

A number of times: In several of the events in the wilderness (e.g. compare Numbers ch.2 to ch.26),

In several events in Judges (e.g. see ch.20),

In several events in the Books of Samuel and Kings (e.g. see 1 Samuel 31:1, and 2 Kings 6:25-30),

At the First Destruction,

In several events during Second Temple times,

At the Second Destruction (Josephus puts the slain at 1,100,000),

At the siege of Betar (see Talmud, Gittin 57a),

During the Crusades,

During the Spanish Inquisition and Expulsion,

During the Chmielnicki massacres (1648-9)

During the Holocaust.

For a fuller listing, see:

http://www.simpletoremember.com/articles/a/HistoryJewishPersecution/