The Bible doesn’t mention exactly when angels were created. It does say in Psalms 8:4-5:
What is man that You are mindful of him, And the son of man that You visit him? For You have made him a little lower than the angels, And You have crowned him with glory and honor.
This suggests that angels were created before man was. Also, Satan had fallen from heaven and caused a rebellion before he appeared in the Garden of Eden to tempt Eve. So, he must have been around for some time previous to the creation of Adam and Eve.
The archangel Michael appears in the Old Testament book of Daniel and in the New Testament book of Revelation in John’s vision of the future, at Armageddon. These events are at least thousands of years apart. Therefore, it seems that angels are eternal since they will be around for Armageddon and there is no reference to angels dying in the Bible.
Neither is there reference to demons dying. In fact, in Jude 6 it says that God has reserved for them “everlasting chains under darkness for the judgment of the great day.” It sounds like they will be chained forever instead of being killed. This makes sense for spirit beings.
All of this makes me wonder if more angels have been created since the initial batch. Since angels help mankind on the earth, maybe as the population of humans have grown, more angels have been created to help them? What do you think?