Most students do 2 sciences (Core science and Additional science). Only rare occasians do people do only 1 science GCSE. Additional science and Core science together means you will have to take 6 exams (last year this was the case). Core science only will be 3 exams. You have the option to do triple science (Biology, Chemistry, Physic) if you're one of the more confident candidates in science (some schools don't offer this though). I did Core science in year 10 and triple science in year 11 so I got 4 GCSE sciences in total.
FYI - 9 exams in total for triple science
(B1,B2,B3 --- Biology GCSE)
(C1,C2,C2 --- Chemistry GCSE)
(P1,P2,P3 --- Physics GCSE)
(B2,C2,P2 --- Addidional Science GCSE)
(B1,C1,P1 --- Core Science GCSE)
This is last year so I'm not sure if any changes were made this year.
Feel free to ask any questions