The versions inserting the "generations" (or similar) after the word "thousands" (אֶלֶף) are providing an interpretive translation. These include: NIV, NLT, BSB, CSB, CEV, GNT, HCSB, ISV, GWT, etc.
The word, generation" (or equivalent) is not in the Hebrew text. However, there is clearly a need to supply some noun to make sense of the "thousands of ... what?" If nothing is inserted in the text, one might mentally insert "people", or, "families", or, "generations", etc; but without anything, the sentence makes little sense to some.
Therefore, many versions supply something, and the most popular choice is "generations", making the statement quintessentially hyperbolic. The main justification for this is the parallel passage that includes "generations" in Deut 7:9.
Other more literal versions leave this blank such as; ESV, NASB, KJV, NKJV, ASV, DRB, WBT, YLT, etc.
My personal view is that, when read correctly, the intent is perfectly clear: the thing that multiplied by thousands is simple - God's mercy to those that love God and keep His commandments. It is this very idea that appears to be the precedent for John 14:15, 15:10, 1 John 5:2, 3, 2 John 6, Rev 12:17, etc. But that is another discussion.