Here the writer contrasts the High Priests in the Old Testament who daily offered sacrifices for sin and the death of Christ as a one-time sacrifice which completely and finally paid for all sin with the fact that people die once and then judgement comes when they die. We are and will be accountable to God at that time. This reference to many does not speak of limiting Jesus’s sacrifice for sin, but is contrasting the one sacrifice and the many who benefit from it as it does in many other places in scripture. When the Lord returns at the Rapture of the Church all Christians will enter into His presence, but not all will receive a full reward after their judgement for how faithful they were here on earth. How many of us will be able say we are saved and I have fought the good fight, I have finished the race and done all that God asked me to do in this world (2 Timothy 4:7–8)?
Hebrews 9:27-28 And inasmuch as it is appointed for men to die once and after this comes judgment, so Christ also, having been offered once to bear the sins of many, will appear a second time for salvation without reference to sin, to those who eagerly await Him.