Are you thinking if the part where it says that two spells would daunt the brain of an ordinary man, and two drive him mad, but through stringent exercise Mazirian could handle up to four of the greatest, or six of the lesser?
That rings a bell, yes.
There is no information at all in that story on the limits of the "greatest wizards" like Phandaal. You must be misremembering.
So, we can presume the maximum is closer to 6 or so, rather than 10. Unless you have a quote about some magician who was able to memorize more?
Is it possible the 'genre' has also moved on from where it was when D&D was first created?
There's no question that we've had more examples of mage protagonists in the genre in the decades following D&D, culminating in stuff like Harry Potter. Maybe it was already heading that way, with influences like Moorcock's Elric, or maybe D&D's profound failure to model the genre even came back and cross-contaminated it. Maybe it's a broad, lasting change, or maybe it's just a new sub-genre.
Even in those cases, though, you don't see anything like D&D's hyper-versatile, fire-and-forget magic-users tromping around a mostly low-magic world as part of a party of other equally plot-destroying casters and supposedly co-equal, yet decidedly marginalized non-casters. Instead, you either see a protagonist who is one among many mages including a bit of an ensemble cast of heroic mages with non-casters relegated to NPC-like support roles or victims (as in Harry Potter), or who has very limited abilities (like Skeeve) who are closer to equal with any non-casting companions, and have to use the same cleverness, luck, and perseverance (and writer fiat) to win through.
Games like Ars Magica and Mage captured the former feel, with non-mages either explicitly-inferior PC options, or not PC options, at all. Games like Fantasy Hero could be used for the latter sort - hmm, and some versions of RQ, I'd think - where there are caster and non-caster PCs, both as viable options.
But, even 4e, which did set up casters & non-casters in a sort of roughly-balanced parity, didn't quite capture anything quite like a definable, extant, fantasy sub-genre. It has elements from many, but it still comes together into an improbable ensemble cast with far too many, and too-disruptive, abilities among them to be suitable 'heroes' in any genre.
Of course, part of it is the problem with an RPG needing to model not only what a genre says it contains, but the plot-power the author exercises. So you have large hp totals and saving throws and the like so the RPG character can survive the 'dangers' faced by a literary hero - yet, the reality of those mechanics in essence all but erase the perceived danger for the player, if they can't suspend disbelief well enough to accept the corresponding genre conventions....