Says the LORD: See 1.8. Good News Translation finds it more natural to move the expression to the end of verse 24.

It is impenetrable … they are more numerous … they are without number: Hebrew Old Testament Text Project takes it as a reference to the forest, and they (twice) as a reference to either the trees (mentioned at the end of verse 22) or the Egyptians (who the trees represent). But as shown by the pronouns used, Good News Translation and most other versions take they to refer to the soldiers of the attacking army; Contemporary English Version expresses this directly by translating verses 22 and 23 together, beginning with “The enemy army will go forward like a swarm of locusts.”

If locusts are unknown, translators can have “swarming insects.”

The whole verse can be rendered:

• Even though the forest is so thick you can’t go through it, they will cut it down. For like locusts, they have too many men to count. I the LORD have declared this.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .