וַיִּקַּ֗ח שֵׁשׁ־מֵא֥וֹת רֶ֙כֶב֙ בָּח֔וּר וְכֹ֖ל רֶ֣כֶב מִצְרָ֑יִם וְשָׁלִשִׁ֖ם עַל־כֻּלּֽוֹ׃
Sechshundert auserlesene Wagen nahm er und alle Wagen Ägyptens, und Wagenkämpfer auf allen.
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.