וַיִּקַּ֗ח שֵׁשׁ־מֵא֥וֹת רֶ֙כֶב֙ בָּח֔וּר וְכֹ֖ל רֶ֣כֶב מִצְרָ֑יִם וְשָׁלִשִׁ֖ם עַל־כֻּלּֽוֹ׃
And he took six hundred chosen chariots, and all the chariots of Egypt, and captains over all of them.
Rashi on Exodus
בחור means CHOSEN. The word בחור is singular number — the idea is: each and every chariot in that number was a selected one.
Sforno on Exodus
וכל רכב מצרים, the chariots of average quality of which there were many.