There's no minimum amount of GCSEs at each grade that you need, but since you want to do medicine and that's one of the most competitive subjects, having mostly A*s and the rest As would be a good start. Most people do 9. 10 or 11 GCSEs. Try to get A*s in the most important subjects, ie maths and the sciences. It may be more important at Oxford because they only interview about 80% of applicants, whereas Cambridge interview almost everyone. You'll need 3 As at A-level and obviously good AS grades will help a lot. Apart from grades, you'll have to write a personal statement, your school will write you a reference, you'll have interviews and you'll have to sit the BMAT, so there are many more areas that are just as important.