Upvote:5
This quota ("лимиты") was introduced in the infamous NKVD Order No. 00447 of July 30, 1937 (yes, as Mark says, Wikipedia does help, you just have to know where to look; Russian Wikipedia mirror is much more detailed than the English link I gave). If and when I have more time, I will add more on this. The only surprising thing about these quotas is that they surprise anybody: After all, USSR was a country of planned economy.