Once created, they never cease to exist.
This is confirmed in the Scriptures which clearly states that angels are not subject to death ((Luke 20:36 "and they can no longer die; for they are like the angels")
Further weight is added to this argument by further Biblical references to the immortality of angels. In Daniel 9, the angel Gabriel appeared to the prophet Daniel.
Then more than 500 years later, this same unaged Gabriel appeared to Zechariah, the father of John the Baptist.
Since angels are immortal and since they do not marry nor have baby angels (Mathew 22:30 At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven) - the numbers of angels in existence will always be the same.
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