וַיִּקַּ֗ח שֵׁשׁ־מֵא֥וֹת רֶ֙כֶב֙ בָּח֔וּר וְכֹ֖ל רֶ֣כֶב מִצְרָ֑יִם וְשָׁלִשִׁ֖ם עַל־כֻּלּֽוֹ׃
y tomó seiscientos carros escogidos, y todos los carros de Egipto, y los capitanes sobre ellos.
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.
Rashbam on Exodus
רכב בחור. armoured vehicles of the best category.