Numbers aren't required. Unless a DM dictates that less than 4 Mind Flayers exist in his world, and that each of these has less than 5 Class Levels, than they cannot threaten the Githyanki Lich-Queen. However, the creation/publication of an adventure in which four 18th Level characters can threaten her existance means that in any world in which four Illithid with at least 5 Class Levels exist that the Lich-Queen's continued existance is as good as forfeit. Since the default for the Mind Flayers is to dwell in cities (plural) of "two hundred to two thousand" (MM, 136), the chances of this are actually more likely than others are willing to admit.
Except that the 2E situation does still exist, as it is described to some extent in the PsiHB (Page 148), almost a direct reflection of the history originally presented for Illithid/Gith in 1E/2E recapped for public consumption in 3E.
Additionally, it distinctly lays out the situation as I have described it: The Illithids are capable of increasing their power over time by means of gaining levels, while the Githyanki are hindered by their Lich-Queen, who consumes those Githyanki that achieve higher than 16th Level (though if this is in Character Levels or Class Levels seems to be a gray spot). At any rate, this presents the problem I originally indicated: By virtue of the level-cap imposed by the Lich-Queen, the Githyanki are effectively limited in power, yet by the new 3E rules regarding level-gain for non-standard races, the Illithids are now able to reach that level and exceed it.
Consequently, for those who have the adventure, it states in the PsiHB ...Besides eliminating potential rivals, the lich-queen enhances her power with the stolen life-essence.
Does the adventure explain this at all, or was it overlooked in development?