There were indeed seven bullocks to be offered at his consecration, one every day, Exodus 29:35,36; but here he mentions only one, either by a common enallage of number, or because he here describes only the work of the first day, and leaves the rest to be gathered from it; of which see Leviticus 8:33.
Related Commentaries of Leviticus 8:14
Leviticus 8:14
14 And he brought the bullock for the sin offering: and Aaron and his sons laid their hands upon the head of the bullock for the sin offering.