Has the number of angels/demons increased since the very beginning? Do they reproduce like humans do or find another way to increase?
The Bible never gives a number to the amount of angels that God created. However, we know that Satan fell from heaven and that one third of the angels went with him (Revelation 12:9, 4).
With Satan and one third of the angels cast out of heaven, that leaves two thirds of the angels to serve God in faithfulness. The Bible has never mentioned any change in these numbers. It seems they are set.
Jesus tells us that marriage in heaven will not happen because we will be like the angels do not marry, nor are given in marriage (Matthew 22:30). We can infer from this that if they do not marry they certainly do not reproduce.
In Genesis 6:1, the sons of God (angels) married and procreated with the daughters of men. But they did not produce angels. Could this have been an attempt by fallen angels to try to reproduce themselves and grow their numbers against God’s superior forces? We don’t know why they tried to do this. Perhaps it was just to be depraved.
Nevertheless, because Scripture does not tell us one way or the other, we can infer that the amount of angels in heaven and in Hell has not changed since the beginning of creation. We don’t have to worry about the devil cheating to try to get more.
Besides this, it wouldn’t matter how many demons served Satan. He has already fallen from heaven and will be cast into the lake of fire at the end of time (Revelation 20:10).