Any time you upload data to your account as a tileset, Mapbox gives it a tileset ID.
Tileset IDs will always start with your Mapbox username, followed by the tileset's unique alphanumeric identifier:
username.identifier. To find the tileset ID for a specific tileset, go to your tilesets page. Click the options menu (the icon) to the right of a tileset's name to find its tileset ID.
Note that styles made in the Mapbox Studio style editor and datasets uploaded to Mapbox Studio or created through the Mapbox Studio dataset editor do not have tileset IDs. Rather, they are identified with style URLs and dataset IDs.