To answer the question, it's As rather than A's.
Don't list them, only list your highest form of qualification.
If you have a degree and or a masters only list them.
No apostrophe's.
Five A grades, four B grades. Not sure if you refer to it as grades, i think it would be better to list each subject and what you got for it. Save them having to ask.
9 GCSE's A-C, done
9 GCSE's A-C, done
5 As and 4 Bs