Numbers 21:6 New English Translation

⁶So the Lord sent venomous^[a] snakes^[b] among the people, and they bit the people; many people of Israel died.

Numbers 21:6 tn Heb “flaming serpents”; KJV, NASB “fiery serpents”; NAB “saraph serpents.” This figure of speech (metonymy) probably describes the venomous and painful results of snakebite. The feeling from such an experience would be like a burning fire (שָׂרָף, saraf).
Numbers 21:6 tn The designation of the serpents/ snakes is נְחָשִׁים (n^ekhashim), which is similar to the word for “bronze” (נְחֹשֶׁת, n^ekhoshet). This has led some scholars to describe the serpents as bronze in color. The description of them as fiery indicates they were poisonous. Perhaps the snake in question is a species of adder.

New English Translation (NET)