What is the significance of the seven bulls and the seven sheep on seven Mizbechos?
Ibn Ezra 1 : 'The number seven is prominent in the days of the week, in the months and in the years; in the seven lambs of the Olah (of most Musaf offerings) and the seven sprinklings (on Yom Kipur). Also Iyov was told to bring seven bulls and seven rams. 2
Ba'al ha'Turim: It was customary for gentiles to bring seven of each species as sacrifices, 3 corresponding to the seven Mitzvos Bnei No'ach.
Rosh (22:41): Hashem does not deny anyone's reward. In all, Balak brought 42 Korbanos; Hashem decreed that 42 Yisraelim die. Moshe prayed that it not be in his lifetime, and told Pinchas, who did similarly. This was pdonkeyed on to Eli, Shmuel, David, Eliyahu and Elisha. Once, lowly people insulted Elisha; he decided that the decree should be fulfilled through them.