he must not multiply horses for himself

"he must not get large numbers of horses for himself" or "he must not keep acquiring more horses for himself'

for Yahweh had said to you, 'You will never return that way again.'

This has a quotation within a quotation. A direct quotation can be stated as an indirect quotation. Alternate translation: "for Yahweh had said that you must never return to Egypt again." (See: rc://en/ta/man/jit/figs-quotesinquotes and rc://en/ta/man/jit/figs-quotations)