Will there be an opportunity for people to repent during the millennial reign of Christ? Will there also be birth and death among both believers and unbelievers alike?
I presume you’re referring to people who will dwell on Earth during the Millennium. The Millennium will begin with only believers who have survived the Great Tribulation and are born again. Since they will still be natural humans they will have children who as they mature will need to decide whether or not to accept the Lord’s death as payment for their sins.
As it is with all natural humans some will accept the Lord and others won’t. Even so, both groups will have children. By the end of the Millennium there will be so many unbelievers that when Satan is released he will be able to recruit a large army from all over the world to mount a final attack against the Lord and His people. But fire will come down from heaven and destroy them all (Rev. 20: 7-10).