A being known to have several Demon Lord class individuals under his/her ruling, that is a Great Demon Lord [大魔王, Dai Maō].
Although it is unknown exactly how many "several" refers to, it can be assumed that one needs to have at least more than 2 subordinates who are Demon Lord class.
A normal Demon Lord class individual is somewhere between EP: 200,000 ~ EP: 400,000. Footman and Tear are just below EP: 400,000 whereas Laplace is more than that.
However, Rimuru Tempest is a Great Demon Lord through and through. He has 14 subordinates who are greater than EP: 400,000 (11 of which are above EP: 1,000,000), and multitudes of subordinates between the normal Demon Lord class range.
Another thing to note is that the title to a Great Demon Lord is updated only when the Great Demon Lord 'candidate' undergoes a great evolution that requires the acknowledgement of the Voice of the World (for example, obtaining an Ultimate Skill).
During the prize giving ceremony after the battle against the Nasca Empire, Rimuru awakened 7 of his subordinates into True Demon Lords (i.e., above EP: 1,000,000; 3 of those above EP:2,000,000) however he was still titled a Demon Lord by the Voice of the World. Only after Raphael evolved into Ciel and upgraded all of Rimuru's skills (probably also coupled with his evolution into an Ultimate Slime some time earlier), his title was updated to Great Demon Lord.
Similarly, Kazaream had all his parameters increased when he was reincarnated as a Seraphim and also obtained the Ultimate Skill Dominion Lord Melchizedek before being recognized as a Great Demon Lord.