Roble, I am afraid no one knows, the number of clans
One possible classification:
Humbannaa-Itu (Harerge highlands )
Arsi
Wallo
Boran-Guji
Maccaa (West Showa, Walleggaa, Illu Abbaa Boora, Jimmaa regions)
Tulama (Showa)
The first three are known as Bareentuma, and others as Borana.
In ancient times, to be Oromo a clan or a person should belong to one of these tribes (or other defunct tribes like the Orma/Warday)
There r numerous intermingling clans and subclans the exact number and classification on which there is no general agreement.
The problem is , in the past, when a subclan gets powerful, it starts to consider itself as a clan!
Besides tribes are free to admit whoever, non oromo clan, they want and thus become powerful;
those admitted can also renegate and revert back to their ancient identities
Arsi and Macca r the two biggest Oromo tribes, they probably each make close to ten millions (Arsi probably a little less).
Once powerful with kingdoms and Sultanates, Wallo oromos r today the smallest oromo tribe, probably hardly a million
B.
So I See You Guys Aren't Like Somalis,
We Are Hardcore To The Bone When It Comes To Our Clans...
We Are Taught Our Lineage At An Early Age...
If Two Somalis Meet, They Always wanna know Which Clan The Other One Comes From....