Depends. Obviously if you are going to apply for Uni in England you need at least two AH's sometimes three. Some courses in Scotland require 2 AH's so double check the entrance requirements for any courses you might want to take.
If you really enjoy a subject and are really interested in it, do carry it on to AH.
Personally I'm taking 3 (well essentially 2 2/3) AH's next year + a higher + an enhancement course. It's up to you and up to what your school offers.
Edit: Why the neg rep "/ ?