According to Netheril: Empire of Magic (N:EoM), at the time of the Netherese there were very few human deities and each one of them carried very many portfolios. It seems like as the millennia went by, those portfolios got shared by many new powers.
For Mystryl, the list went like this: Magic, spells, the weave, wizards, spellcasters, energy, creativity, knowledge, invention, song, time, spring (page 49 of The Winds of Netheril).
We can compare this list with the portfolios given in the FR Adventures (FRA) and Faiths & Avatars (F&A) sourcebooks. We see that every portfolio except Time is covered: Azuth (mages, energy according to FRA; wizards, spellcasters according to F&A), Oghma (knowledge according to FRA; knowledge, invention according to F&A), Lathander (spring according to FRA; spring, creativity according to F&A), Milil (song), and of course Mystra (magic, spells, the weave).
Given that every other one is somehow covered, and the great many commonalities between N:EoM and F&A, I would agree with Carcer's statement and say Time appears to have been intentionally left out by TSR. (F&A lists Julia Martin with Eric Boyd as designers. For N:EoM, Julia Martin is listed under "additional design" and Eric Boyd is listed as a proofreader.)
There is one final curiosity: Amaunator is mentioned (in both N:EoM and F&A) to have misinterpreted some godly contract and wanted to preside over passage of time, but decided not to interfere with Mystryl. It looks like after Mystryl's demise, he still did not claim Time. So perhaps Mystra could still be the "hidden inheritor" of the portfolio and might someday claim it openly. It might also be worth mentioning that three "time gates" that allow characters to travel back to the Netherese period were still active as of 1370DR, and they were being protected by Mystryl's magic (N:EoM). It would be an interesting scenario to have Midnight/Mystra sending an avatar/proxy to meet her old self, Mystryl.
On a related note: Here is a list of powers from F&A who grant major access to the sphere of Time to her/his speciality priests: Amaunator, Auril, Chauntea, Cyric, Eldath, Kelemvor, Leira, Mask, Mielikki, Myrkul, Mystra, Shar, Silvanus, Sune. We broadly see that they are mostly deities of nature, death, or deception. If you really want to assign the portfolio of Time in your own game, consider one of these.
Deities followed by chronomancers
There is an article with the title Chronomancy & the Multiverse, version 1.1 by Roger E. Moore, published as "an updated and expanded excerpt from the Chronomancer", a 2e accessory. The article was made public by the WotC on their free downloads for previous editions site. This article has a section of the effects of chronomancy in the Realms, and relates time with deities of knowledge and writing. It is stated:
Native chronomancers are usually devout followers of both a deity of magic (e.g., Mystra) and a deity of time or history (e.g., Deneir or Oghma), serving as historians and information collectors. Elven chronomancers of Labelas Enoreth are possible. Shou Lung, in Kara-Tur, might have a secret, officially approved group of chronomancers, probably Historians, working for the Emperor.
Paraphrased from the Dragons of Faerun sourcebook:
Long before the Crown Wars, there was the Age of Dragons. Dragons ruled over all other races (their foodstock) with impunity. The elven high mages, working in secret in the northern glacier, weaved a giant mythal over all of Faerûn: The Dracorage Mythal.
This mythal would induce insanity and rage upon the dragons, causing them to attack and destroy anything in their path. This included their lairs, offspring, followers, anything.
Reduced to mere rampaging beasts, the dragons could no longer rule absolute over all other races; they had won a fighting edge over the dragons.
The dracorage mythal is linked to the appearance of the King-Killer Star. Depending on the position of the comet in the sky, the rage can be localized or global.
In 1373 DR, the link between mythal and comet was broken:
Hammer 1: Sammaster successfully ties his phylactery to the Dracorage mythal, separating the Rage of Dragons from the King-Killer Star and effectively causing all dragons to go mad. Sammaster is later destroyed (on Nightal 6), ending the Rage of Dragons.
The dracorage mythal still exists, but now the only way to trigger a rage is by using the extremely rare 4th level spell Aggravate Dracorage. This spell only exists in 3.5e though.
Best Answer
There is no 100% consistent answer.
I’ve since bought the book online and read parts of it. Evermeet: Island of Elves explicitly mentions hundreds of high mages (not just regular ones) gathering for the spell:
That book came out in April 1998.
Which means it comes after Cormanthyr: Empire of Elves, which came out in march 1998 and only mentions 12 high mages.
Finally, Mordenkainen’s Tome of Foes came out in May 2018, and also mentions 12 high mages.
Conclusion: Since two official sources both mention 12 high mages, including the most recent mention of a number, and the only mention of hundreds is in a stand-alone novel (not an official source), I believe it is safe to conclude that 12 high mages is enough to catastrophically damage a planet.