I, Ezra, saw on Mount Zion a great multitude, which I could not number: Compare Rev 7.9. Ezra was not on Mount Zion himself, but looking toward it. So we may render this clause as “I, Ezra, looked over toward Mount Zion, and saw so many people there that I could not count them” or “… and saw a large group of people. There were so many I couldn’t count them.” Mount Zion is the hill in Jerusalem where the Temple stood.

And they all were praising the Lord with songs may be translated “All of them were singing praises to the Lord” or “… singing songs telling the Lord how great he was.” Compare Rev 4.11.

Here are possible models for this verse:

• I, Ezra, looked over to Mount Zion, and saw so many people there I could not count them. All of them were singing praises to the Lord.

• … and saw a huge number of people there. There were so many that I couldn’t count them. They were all singing songs telling the Lord how great he was.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Esdras. (UBS Helps for Translators). Miami: UBS, 2019. For this and other handbooks for translators see here.