I would agree with others who have said that the additional maths, and triple science aren’t really much more work at all for a child in the top sets, so I wouldn’t worry about those, they should be v doable (the only annoying thing is a few more maths exams at gcse time). But I don’t think that’s your concern really - just adding another voice to the “that should be fine” chorus.
So your school require that as well as the English maths and science, they take:
1 language
1 humanity
1 RE
2 other GCSEs of any sort.
I agree that seems too much to require of everyone. Usually when schools require gcse RE it is one of the 3 or 4 gcse options on top of the maths/engl/science. But I agree with others that since this is how they timetable it, it’s highly unlikely they’d allow a child to drop out of a timetabled slot unless they were really struggling academically. I’m afraid I have no experience at all of withdrawing from RE, but I’d keep asking around. Would your child be ok doing that?
I would say though, that for both of mine they wanted to take as many as possible as they both struggled to decide what to drop. And actually they both did loads in the end (though RE wasn’t compulsory and only one of mine took it).
One did 12 GCSEs (2 Eng, 3 science, maths, and 4 options, plus gcse statistics and “gcse” further maths). The gcse statistics was taken as one after school class per week by choice by top set students (not obligatory) and they sat it in y10, then the gcse further maths was just done in normal lesson time as they got through the material faster. So 1 gcse a year yearly and 11 in y11. He did really well with mostly 9s.
Second child did the same as the first re the statistics and FM, but also sat an additional gcse in astronomy as an after school class once a week too. He was v keen to do it because it was v interesting and the teacher was great. So he did 1 GCSE in y10 and 12 in y11. He also got loads of 9s and 8s. There were a lot of exams though!
This was this summer, and two summers ago at a comprehensive school in south of England. I know they are very flexible and while they enabled mine to do 12/13 GCSEs, they have friends with SEN or who were just really struggling who were totally ok to work with a customised timetable and sit what they could (baseline for them would have been 9, but I think they were allowed to drop some to take what they could manage).
They neither of them needed that many. But they did take all the extra maths in their stride, and for my eldest who ended up doing maths and FM at a level and getting A star in both and is now doing engineering at uni, the faster pace of maths at gcse was pretty good practice for the intensity of maths and FM a level - though I don’t think absolutely necessary. Probably useful though if he’s going to stay on at the school and everyone else will have also done the extra maths stuff.
And for the extra options - they just really were keen to do all those subjects. Each still had subjects they were sad to drop. A levels are so narrow and a lot of joy and interest came from the option slots at gcse. But I think our schools way of offering them as extras, rather than requiring them as timetabled for everyone, is a better one. Wishing you luck.