Hebrews 7:23

The [former successive line of] priests, on the one hand, existed in greater numbers because they were each prevented by death from continuing [perpetually in office];

1 Chronicles 6:3-14

The sons of Amram: Aaron, Moses and Miriam. And the sons of Aaron: Nadab, Abihu, Eleazar, and Ithamar.

Nehemiah 12:10-11

Now Jeshua became the father of Joiakim, Joiakim of Eliashib, Eliashib of Joiada,

Hebrews 7:8

Furthermore, here [in the Levitical priesthood] tithes are received by men who are subject to death; but in that case [concerning Melchizedek], they are received by one of whom it is testified that he lives on [perpetually].

Treasury of Scripture Knowledge

Summary

And they truly were many priests, because they were not suffered to continue by reason of death:

General references

Bible References

Were

Hebrews 7:8
Furthermore, here [in the Levitical priesthood] tithes are received by men who are subject to death; but in that case [concerning Melchizedek], they are received by one of whom it is testified that he lives on [perpetually].
1 Chronicles 6:3
The sons of Amram: Aaron, Moses and Miriam. And the sons of Aaron: Nadab, Abihu, Eleazar, and Ithamar.
Nehemiah 12:10
Now Jeshua became the father of Joiakim, Joiakim of Eliashib, Eliashib of Joiada,

General references

Numbers 20:26
and strip Aaron of his garments and put them on Eleazar his son. So Aaron will be gathered to his fathers, and will die there.”
Zechariah 1:5
“Your fathers, where are they? And the prophets, do they live forever?