Question: "Is it possible that more of the angels could sin?"
First Timothy 5:21 tells us, “I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favoritism.” No matter what view you take on election, the Bible is clear that God was somehow involved in choosing who would be saved—or, in this case, which of the angels would not sin.
God’s sovereign choices are seen throughout the Bible: He chose Abraham to be the father of many nations (Genesis 17:4-5); He chose Israel to be His special people (Genesis 17:7); He chose Mary to be the mother of Jesus (Luke 1:35-37); He chose the twelve apostles to live with the Lord Jesus for three years and learn from Him (Mark 3:13-19); and He chose Paul to bring the gospel to many people, both personally and through his writings (Acts 9:1-19). In the same way, He has chosen people “out of every tribe and language and people and nation” (Revelation 5:9) to come to faith in Christ. Those whom He chooses will come to Him, and He will never cast them out.
It seems that God also made a choice concerning the angels. God’s holy angels are “elect” – meaning that God has chosen them. Perhaps God gave all of the angels a one-time choice to obey Him or not. In any case, those that sinned and followed Lucifer are lost and condemned. Those that chose to remain loyal to God are secure in that decision. The Bible gives us no reason to believe that it is possible for more of the angels to sin, any more than it gives us reason to believe that the elect will fall away and be lost forever.