As others have said, she's essentially massively wasting her time. There's nothing wrong with smart kids doing extra studies on the side if they want to, but what she's doing seem less directed towards any enjoyment or developmental benefit and more to just feel superior and/or to have something to boast about.
There are decent reasons for a kid like this doing a few extra subjects in their own time. Maybe just because there's a subject they enjoy which their school didn't offer. Maybe because they want to do a particular niche degree that means they need some additional subjects. Or even simply because they can't make up their mind about what degree they want to do and want to keep their options open. But none of these reasons seem to apply here. Despite mentioning wanting to go to Oxford, there's no mention of what she plans to study there (this reinforces my aforementioned sense that this is all more about boasting or feeling superior than real academic development or enjoyment) and she's doing many totally unnecessary subjects regardless of what degree she goes for (as others have mentioned, there's significant overlap and some of these subjects are not particularly well-regarded). Nor does she particularly seem to enjoy the subjects if she's rushing through them in only ten days each.
So, why is she taking them? My guess is that, as with taking all the school entrance exams, she'd claim it was “just for intellectual challenge”. But the reality is that she isn't really intellectually challenging herself that much. In theory, GCSEs across subjects require a similar level of skill, as do A Levels. So having more of them doesn't demonstrate a higher level of skill, just the same level of skill across a wider range of fields. If you found the Maths A Level easy and unchallenging, there's no real reason to think you won't find the Statistics A Level similarly unchallenging.
But surely it's challenging to do so many in such a short space of time, right? Eh, sort of, but still not necessarily that much. Being able to complete a course intended to take two years in just ten days demonstrates an impressive degree of industry as well as information absorption and memory skills, absolutely. But you've really proved that in the first two or three times you do it, you don't need to do it another 20 times on top of that. By the time you come to your 17th 10-Day GCSE in a row, having successfully scored the top grade on each of the preceding 16, what's the challenge? You know you can do it already, you really should just be finding it very easy by that point. Furthermore, ability to absorb information isn't necessarily the same as ability to evaluate, reason, or problem-solve.
The obvious question is, instead of having spent all her time studying for extra GCSEs, why didn't she study for A Levels back then instead of now, especially given she claims to have been able to do A Level standard Maths at age 8? Why is she spending all her time studying for extra A Levels now rather than - as others have suggested - trying to do a correspondence degree or some other more advanced course? She complains that her schools wouldn't let her move up to more advanced classes, yet she's repeatedly choosing to limit herself to less advanced courses.