@kuzunagi13 not quite, there are two kinds of Demon Lords; powerful beings that have been "acknowledged" as a Demon Lord by other Demon Lords, but they're usually weaker than the "True" Demon Lords that have ascended to the class of Demon Lord by meeting specific requirements. Think of it like people that call themselves Hero, vs the True Hero ordained by the world, but for Demon Lords instead (strictly speaking, this also happens to Heroes in this world). The Orc Disaster, for example, could theoretically have been strong enough to become acknowledged as a Demon Lord, but was actually trying to fulfill the conditions to ascend into a true Demon Lord.
Clayman, Karion and Frey are Demon Lords of the first kind, as they refer to themselves as Demon Lords, and deemed to not disgrace the name. Leon Cromwell is acknowledged as a Demon Lord, although strictly speaking as a former True Hero his power is more in line with the True Demon Lords than Clayman and co. Milim and Ramiris are both True Demon Lords. Yes, *that* Ramiris.
Strictly speaking, as of now Rimuru is still just a Slime, albeit one blessed with a name by the Storm Dragon. That changes later though and the Web novel translation is finished, so check that out if you wanna spoil yourself.