Hello!
Okay so let's start with GCSEs. On average people will take 10 GCSEs. Out of these 10 you will have to take: Mathematics, English Literature, English Language, Biology, Chemistry, Physics, a language of your choice (the choice selection depends on the school you attend), a humanity of your choice (usually between History, Geography, Religious Studies), option no.1 (again the optional subjects you take are your choice but are dependent on what school you go to), and option no.2.
In terms of A-Levels, people on average will take 3 but you can also apply to take 4 if you have high enough grades within your GCSEs. A-Level choice is dependent on both your choice and what GCSEs you have taken, some subjects may not be available to you if you do not have the relevant GCSE.
In the UK A-Level study takes place in both Year 12 and Year 13 (these years are what we call Sixth form, Year 12 being 'lower sixth' and Year 13 being 'upper sixth'). The actual A-Level exams take place at the very end of Year 13, meaning you would have been studying 2 years before you take the exam.
I hope this has helped you and wish you luck in what your goals ~