Or HaChaim on Numbers 7:87/אור החיים על במדבר ז׳:פ״ז

1 כל הבקר לעולה, All the oxen used for the total-offering, etc. The word בקר is in the singular as each one formed part of the same collective offering. This is also the reason the Torah used the word זבח השלמים in verse 88 describing the sacrifice as one.