All who are going.
In describing those in heaven, John said, "Then I looked, and I heard the voice of many angels around the throne, the living creatures, and the elders; and the number of them was ten thousand times ten thousand, and thousands of thousands" (Revelations 5:11). John is saying, imagine the biggest crowd of people you can, and the number in heaven is far, far larger than that.
God promised Abraham, "'Look now toward heaven, and count the stars if you are able to number them.' And He said to him, 'So shall your descendants be'" (Genesis 15:5). An interesting thing is that we still don't know how many stars exist in the universe. Just when someone thinks they have a good estimate, technology improves and we find there were more than thought previously. Paul informs us that God wasn't just talking about physical descendants. "Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all" (Romans 4:16). So the writer of Hebrews concludes, "Therefore from one man, and him as good as dead, were born as many as the stars of the sky in multitude--innumerable as the sand which is by the seashore" (Hebrews 11:12).
And even then, it could hold more, but most people will refuse God's offer. "Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it" (Matthew 7:13-14).