Numbers that are huge and precise are data, information, something quite different.
The thing with illegal primes is that they are small enough to really feel like numbers rather than a concatenation of things forced to look like a number (“the shape of my bedroom is 201809!”)
I think there is a biggest number though, and it should be higher than 64bit-1 because your processor would otherwise occasionally handle illegal numbers, thereby (probably) making it illegal as well.
I think the better point to make is that the law will never actually produce a concrete N, preferring to leave itself vague so that it can criminalise things at a whim. This is what makes it a farce and why such notions need to be purged from the law.
For natural numbers, the lawyers seem unable to do even that. Sure, they might want to name some of the "illegal primes" mentioned in this thread as examples, but what will they say about those numbers +1? They're gonna paint themselves into a corner of ridiculousness nomatter what.
What does "in real life" mean? You and your brain presumably exist in real life. If you brain can construct any natural number, then do they not exist just as much as the ones that happen to appear as counts of rice or the price of beer?
One might stretch the definition to allow numbers that you could read on a postcard in one go to still count as numbers, but it’s a big stretch. If there’s a more understandable representation of the number — especially if there’s a more understandable representation — then it’s data and not a number.
Beyond a certain amount of information content numbers just become data. Certainly at the point where counting the number of digits is non trivial. (There are notations to make magnitude easier to see, but these purposefully delete information content.)
But this will, in time, generate every copyrighted material.
When framed this way, so many laws become ridiculous. Copyright is about establishing a monopoly on numbers. It's illegal for normal people to know certain numbers because they represent classified information. People go to jail because of numbers stored in their hard drives.