Exodus 14:7

בחור means CHOSEN. The word בחור is singular number — the idea is: each and every chariot in that number was a selected one. וכל רכב מצרים signifies AND with these were ALL THE other CHARIOTS OF EGYPT. And whence came all these animals required for the chariots? If you say that they were from the cattle of the Egyptians — but you know it is stated, (Exodus 9:6) “And all the cattle of the Egyptians died”. And if you say that they were from the Israelites’ cattle — but is it not stated, (Exodus 10:26) “Our cattle, also, shall go with us”! Then whose were they? They were of those “who feared the word of the Lord” and saved their cattle by bringing them into their stables (Exodus 9:20). Deriving it from here (on account of this fact) R. Simeon said: The best amongst the Egyptians — kill him (otherwise he will afterwards devise evil against you); the best amongst the serpents — crush its brains (Mekhilta). ושלשים על כלו means army-captains, as the Targum has it.

וַיִּקַּ֗ח שֵׁשׁ־מֵא֥וֹת רֶ֙כֶב֙ בָּח֔וּר וְכֹ֖ל רֶ֣כֶב מִצְרָ֑יִם וְשָׁלִשִׁ֖ם עַל־כֻּלּֽוֹ׃

and he took six hundred chosen chariots, and all the chariots of Miżrayim, and captains over every one of them.