Think about it like this: the galactic filament realm is like a bucket. The bucket gets larger as the number of galaxies gets larger. Primordial essence are like bricks, you can only put so many bricks in a bucket at a time. After each milestone the bucket gets bigger and allows for more bricks. In the end, each stage is still a bucket, just of a different size. Just like how a Monarch and a Paragon are still in the Galactic Filament realm, just with different capacity for primordial essence refinement.

Like Adui said, Infinity isn't a number. Infinity is a concept. So something like 2 * ∞ still equals ∞. But something like ∞ / ∞ is an indeterminate, meaning we don't know what it is, unless there's more context (has to do with derivatives and whatnot). So (2 * ∞) / ∞ is equivalent to ∞ / ∞, or indeterminate. If you want a better understanding, look up Hilbert's Hotel Paradox, it's a good way to conceptualize infinity and why something like ∞ / ∞ is indeterminate. But 5% of infinity is still infinity lol, since 0.05 * ∞ = ∞. But can't have everyone being broken, I guess.