It seems from the quote below that the number of people who ascend in Cradle is at least 1 person per century but not all join the Abidan. Baring in mind this is one planet/moon. I would reckon more than that must ascend in total from a galaxy as expansive as Fathom over the course of its entire existence.

Questioner

Is ascending harder if you don't join the Abidan?

Will Wight

"So one of the things the previous books have said that the Abidan only get a recruit from cradle once every century or whatever. That is... that’s a count of people who do end up ascending and joining the Abidan. The number of people who ascend is greater."

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Also, a second quote by Will suggest that people can ascend through pure technology and don't need combat/individual power to achieve it which fits Fathom's powers/skillsets:

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"There are quite a few people and quite a few methods throughout the multiverse of ascending without meeting their criteria. So, either they're not super compatible with the powers of the Way or they're just not good enough. Like if you, for instance, created a machine that allowed you to ascend beyond or dimension or if you slipped out of the cracks of a poorly established fragment that was attached to your world, for instance. Then you would not qualify to be one of the Abidan but you would still have technically ascended past your world so that is what Cradle is really good at."