Some angels did rebel
The Bible is clear that angelic rebellion is not merely imaginary. Peter writes of angels who sinned, and Jude describes angels who abandoned the place God had assigned to them. These passages point to a real, terrible turning away from God, associated elsewhere with the devil and his angels (2 Peter 2:4; Jude 1:6; Matthew 25:41).
That means angels were not created as mindless forces. They are personal servants of God, able to worship, obey, and, in the case of some, rebel. Revelation portrays the devil leading other angels in opposition to God, though their defeat is certain (Revelation 12:7-9).
For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
The Bible doesn't give a direct prediction
Could an angel who has not rebelled yet sin in the future? The Bible simply does not directly answer that question. It never says, in so many words, that every faithful angel is now incapable of sinning. Nor does it predict another angelic rebellion.
Paul refers to "elect angels" when charging Timothy before God and Christ (1 Timothy 5:21). Many Christians understand this to mean angels whom God has chosen and preserved in faithful service. That is a reasonable reading, though Paul does not stop to explain the phrase in detail.
God's faithful angels serve him
The angels we meet in Scripture who remain with God are shown carrying out his will. They rejoice when sinners repent, minister to those who belong to Christ, and worship God before his throne (Luke 15:10; Hebrews 1:14; Revelation 7:11-12). Their attention is not on building their own kingdom.
Jesus also speaks of angels in heaven as belonging to the life of God's coming kingdom, not the fallen patterns of this world (Matthew 22:30). Scripture directs our focus less towards guessing at unseen possibilities and more towards the steady reality that God rules over every spiritual power (Colossians 2:10).
Evil will not have the last word
Whatever questions remain about angels now, the ending Scripture gives is wonderfully firm. The devil will face final judgement, and God's renewed creation will be free from the curse of sin (Revelation 20:10; Revelation 22:3). God will not leave his world vulnerable to rebellion forever.
So it is fair to say, "I don't know whether more angels could sin." It is even more certain to say that no rebellion can overturn God's purpose. The final future belongs to Christ, and all evil, visible and invisible, will answer to him.
And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.