In the mountains, i.e. in or at Mount Sinai, the plural number for the singular; or, in this mountainous desert.
Related Commentaries of Exodus 32:12
Exodus 32:12
12 Wherefore should the Egyptians speak, and say, For mischief did he bring them out, to slay them in the mountains, and to consume them from the face of the earth? Turn from thy fierce wrath, and repent of this evil against thy people.