Advice at DDs school is def 3 A Levels (4 if FM) and you could be considered a competitive Oxbridge candidate if you can meet one of 2 criteria. Either
(a) 8 or 9 GCSEs at A star plus a prediction of 1 x A star and 2 x As
(b) 4 or 5 A stars and the rest as As (possibly the odd B) with the As in subjects that are not related to degree subject (eg All A stars in sciences and maths if you are doing Physics) plus 2 x A star and 1 x A predictions for A Level.
Sorry Sos, I can't remember your Dgds gcse results but if they are brilliant then it sounds like 1 x A star and 3 x A could be OK?
....you also need to be in the top handful in the school year. Apparently they report this on their UCAS references (not sure whether this is standard?). Obviously you would also need to nail the aptitude tests and get a good school reference/recommendation too.
They also said Oxbridge throw massive curve balls every so often so it's worth applying if you have a stellar PS and can demonstrate that you have a massive spike in your profile (eg maths genius).
This is at a school where contextual offers are very very unlikely. Do Oxbridge adjust their grade expectations for those from other demographics?
....in the real world I am not sure Oxbridge is always the best Uni for all subjects anyway. LSE and Imperial are def up there. Last year one girl at DDs school declined Oxford and went to LSE to do Politics instead.
I am very very far from being an expert in any of this - this is just what we are being told
DD has an 'outside chance' but has been advised to start preparing so she can give it a whirl subject to her predictions at the end of the year (she would need 2 x A star predictions to get the schools backing to enter what can only be described as the application lottery)
So based on what I have been told I would def agree with the other OPs advice that 3 really good ones are probably better and the lowest risk option.