MILLIONS of celestials? Celestials win.
In the point about Ion being greater than Eternity, that doesn't mean Ion is going to take out Millions of beings that are greater than cosmic cubes.
Points why:
1) We don't know how many Exitar level celestials there are. There could be more, and there probably are more.
2) Think about how many cosmic cubes it took for The Goddess or Magus to try and take over a universe. Not that many. Celestials are known to work in concert. Cosmic cubes have universe destroying potential, they just limit themselves. It was stated that cosmic cubes have infinite potential, boundries are set during their creation and evolution.
3) During the comics where it showed M-bodies being created, if you look close there is a set of Celestial legs there. The Celestial's physical form that we associate with them could merely be M-bodies, not their true form. It has been hinted at that they existed in the universe before 616 that collapsed and gave it's power to Galactus. Their age may possibly be greater than that of the universe itself. During many cosmic threats, certain representatives of the Celestials seem to show up with no explanation, further leaning into the possibilities that the Celestials are using M-bodies.
4) Eternity when mentioning the Celestials states: "And from this meditation came the manifestation - as the Celestials appeared from the nebulous spheres. " From the wording of that it sounds as if Eternity summoned the Celestials to him as opposed to creating them, as it in no way shows any kind of birth, it only shows in the scan fully developed Celestials appearing:
http://www.geocities.com/brenni_au/Surfer.jpg
5) The cosmic cube beings Kosmos and Kubik — beings who wield nearly incalculable energy, matter and reality manipulation powers — stated that a single Celestial possesses "power many orders of magnitude beyond our own."
6) It has been suggested by Reed Richards that the Celestial's source of power is Hyperspace itself — the source of energy in the Marvel Universe. Beings like Hyperstorm were merely tapping into a portion of what the Celestials use as energy. A source of energy that potentially greater than a single Eternity, maybe tapping into Multi-Eternity directly. This would place them on par with a very similar energy to what Ion wields - if it came down to it, they may have more experience with using it, and the battle would ultimately take place outside of regular time and space as Celestials are multi-dimensional entities, similar to 5th dimensional imps.
While Ion is far more powerful than a single Celestials, when he is dealing with millions of them at once, it is going to be no easy matter. More likely than not, if the Celestials were to change course and attempted to take over the universe, they would do so with ease. To top it off the Celestials are said to be responsible for the creation of Omega mutants. Beings like Jean Grey (whitecrown phoenix) and Franklin Richards seem to have no upper limit to what they are capable of, and have been hinted at as replacing the cosmic entities themselves, including Eternity, Death, etc., which implies that the true power of the Celestials are collectively greater than that of a single universe.
By saying similar to 5th dimensional imps, to clarify, I mean in that they exist in more dimensions than normal comics take place, not meaning that they have the same powerset as Mxy.
Keep in mind also that Ashema held an entire universe inside of her.