Numbers 7:16

שעיר עזים אחד לחטאת ONE KID OF THE GOATS FOR A SIN-OFFERING, to make expiation for uncleanness caused by a grave in the depths of the earth, (i.e. one that is not known to exist and it might therefore be assumed that people passed over it and thereby unwittingly became unclean), which was only a doubtful uncleanness (one about which there exists a doubt as to whether it actually has been incurred or not) (cf. Sifrei Bamidbar 51).

one kid of the goats for a sin offering:

שְׂעִיר־עִזִּ֥ים אֶחָ֖ד לְחַטָּֽאת׃