Question: How many Jehovah's Witnesses were forced to wear a purple triangle?
About the article "The Holocaust", section "Jehovah's Witnesses".

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Refusing to pledge allegiance to the Nazi party or to serve in the military, roughly 12,000 Jehovah's Witnesses were forced to wear a purple triangle and were placed in camps where they were given the option of renouncing their faith and submitting to the state's authority.
Between 2,500 and 5,000 were killed.
Historian Detlef Garbe, director at the Neuengamme (Hamburg) Memorial, writes that "no other religious movement resisted the pressure to conform to National Socialism with comparable unanimity and steadfastness."
Jehovah's Witnesses came into conflict with the Nazi regime because they refused to salute Adolf Hitler with the traditional "Heil Hitler" salute, believing that it conflicted with their worship of God.
Because refusing to salute Hitler was considered a crime, Jehovah's Witnesses were arrested, and their children attending school were expelled, detained and separated from their families.
When Germany made military enlistment mandatory, Jehovah’s Witnesses were persecuted because they refused to bear arms.
Being politically neutral, they also refused to vote in the 1936 elections.
Based on Nuremberg Laws, Jehovah's Witnesses who were also classified as ethnic Jews wore a badge comprising a purple triangle superimposed on a yellow triangle. 
  Red triangle—political prisoners: social democrats, socialists, trade unionists, Freemasons, communists, and anarchists.
Green triangle— "professional criminals" (convicts, often working in the camps as Kapos).
Blue triangle—foreign forced laborers, emigrants.
Purple triangle—primarily Jehovah's Witnesses, and members of other Bible Student groups.
Pink triangle—primarily homosexual men, as well as sexual offenders including rapists, paedophiles and zoophiles.
Black triangle—people who were deemed "asocial elements" and "work shy" including * Roma (Gypsies), who were later assigned a brown triangle * The mentally ill * Alcoholics * Vagrants and beggars * Pacifists * Conscription resisters * Lesbians * Prostitutes * Some anarchists * Drug addicts  Brown triangle—Roma (Gypsies) (previously wore the black triangle).
Uninverted red triangle—an enemy POW, spy or a deserter.
Single triangles.
People who wore the green and pink triangles were convicted in criminal courts and may have been transferred to the criminal prison systems after the camps were liberated. 
The purple triangle was a concentration camp badge used by the Nazis to identify "Bibelforscher" (the German name for Jehovah's Witnesses and other Bible Students congregations) in Nazi Germany.
A small number of Adventists, Baptists and pacifists (less than one percent) were also identified by the badge.
Nazism opposed unorthodox-Christian religious minorities (along with Jews), but made the Bible Students the object of particularly intense persecution, including such extensive incarceration that a distinct badge was assigned to them. 
In 1934, the Watch Tower Bible and Tract Society published a letter entitled "Declaration of Facts".
However while the Jehovah's Witnesses sought to reassure the Nazi government that their goals were purely religious and non-political and expressed hope that the government would allow them to continue their preaching, Hitler still restricted their work in Nazi Germany.
After this J. F. Rutherford began denouncing Hitler in articles through his publications potentially making the plight of Jehovah's Witnesses in Nazi Germany worse.
Jehovah's Witnesses or "Bible Researchers" (Bibelforschers) as they were known in Germany, comprised 25,000 members and were among those persecuted by the Nazi government.
All incarcerated members were identified by a unique purple triangle.
Some members of the religious group refused to serve in the German military or give allegiance to the Nazi government, for which 250 were executed.
An estimated 10,000 were arrested for various crimes, and 2,000 were sent to Nazi concentration camps, where approximately 1,200 were killed.
Jehovah's Witnesses were among the few who could leave the concentration camps simply by signing a document renouncing their religious beliefs.