5 igcse’s is the minimum requirement number, many applicants will have more. It may seem good, or normal, at your school, so context will be important, but just be aware it’ll look minimal comared to students with 8-10, which is very common for students who apply to US universities.
UC’s don’t distinguish between A and A* for GPA conversion AND admission (two different things).
For instance, they clearly state they just want “A grades” for everything, no distinction between A and A*, but do not consider igcse’s sufficient, so ask them whether igcse’s + 1 year of APs is sufficient (unless you have 2years, because they want to see 2 years after igcse’s).
“Students who have completed the IGCSE or O-level exams have not yet met the requirements for admission evaluation at UC Berkeley. The university requires further study, either completion of the 2-year A-level program with a minimum of 3 academic exams, the IB diploma program, or another similar academic curriculum. (Most A-Level students who apply to UC Berkeley anticipate receiving A marks on their exams.)”
http://admissions.berkeley.edu/internationalrequirements
or
Students who have attended school under the British system must present five GCSE/Ordinary-Level examinations and at least three Advanced-Level examinations with superior grades. Results of Advanced-Subsidiary or Higher-Level exams should also be submitted.
https://www.admissions.ucla.edu/prospect/intl.htm
However you also listed Princeton, MIT, etc… and those do like to see AAA or AA*A at A Level (as in “nudge”, not “hook”) for admission purpose.
Malcolmx99 refers to “Option2”, which is specific to MIT.
http://mitadmissions.org/apply/international/intltests