It varies. At my high school the general number was 10-11 depending on if you did 1 or 2 English GCSEs (English, vs English Language and English Literature separately). I did 12 total, with the 12th being done through an afterschool club (although it ended up being half self-taught). There were also other exceptions, like people who already speak a language being able to take that GCSE - e.g. a couple of people in my year spoke Portuguese and were able to take the GCSE even though the school didn't actually teach it.
You have to English, maths, and some sort of science wherever you go, I think. Some have other requirements, e.g. obligatory RS/RE, ICT, or languages. Then you can choose some number of options, e.g. history, geography, music, art, etc.