Numbers 28 lists the musaf offerings of rosh chodesh. They include a goat, which is a chatas offering, and several animals as an ola offering.
Numbers 29 lists the musaf offerings of Rosh Hashana. They include just one goat, which is a sin offering. They also include by reference (in verse 6) the ola offerings of rosh chodesh, since Rosh Hashana always coincides with rosh chodesh. But they don't mention rosh chodesh's goat. Why not?
(Note that the rosh chodesh goat is offered on Rosh Hashana, as we see detailed in the musaf prayer. But it's not mentioned in Numbers.)