Tafsiran Bilangan 7:87: Or HaChaim & Rabbeinu Bahya

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.

כל הבקר לעולה, “all the cattle for the burnt-offering, etc.” This offering of the princes on the occasion of the inauguration of the Tabernacle comprised 12 bulls, 12 rams, 12 male sheep, 12 he-goats corresponding to the twelve tribes in the celestial spheres.

You are aware already that the attribute כבוד which descended on Mount Sinai was the same which now had taken up residence in the Tabernacle. Just as at the time this attribute descended onto Mount Sinai it was accompanied by myriads of ministering angels, so the journeys of the Jewish people with the Tabernacle took place under the auspices of flags. Just as at Mount Sinai G’d had warned the people not to touch the mountain (Exodus 19,13), a similar restriction was placed against entering the sacred domain of the Tabernacle when the Torah writes: “any stranger (non-priest) who approaches shall die” (Numbers 1,51).
Here too during the inauguration of the Altar and the Tabernacle, we have a replay of some of the events which occurred at the revelation at Mount Sinai. Twelve sacrifices were offered on this occasion, corresponding to the 12 monuments on behalf of the twelve tribes of Israel which Moses had erected in Exodus 24,4. On this occasion 24 bulls were offered (as peace-offerings).