וַיִּקַּ֗ח שֵׁשׁ־מֵא֥וֹת רֶ֙כֶב֙ בָּח֔וּר וְכֹ֖ל רֶ֣כֶב מִצְרָ֑יִם וְשָׁלִשִׁ֖ם עַל־כֻּלּֽוֹ׃
И взял шестьсот избранных колесниц и все колесницы Египта и всех начальников.
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.