Mine both took five. I have just checked and the majority at their school take four, though some will drop down to three at the end of year 12.
The extra, which is still available, comes from electronics which is an option as part of the Yr 12 "cultural perspectives" programme, where students were encouraged to keep some unexamined breadth: a language, music, art, take up sign language, or an EPQ. Electronics was the exception as they worked towards a Welsh board AS. And if they wanted, could keep it up through A level. The teacher was popular and the emphasis was on building course work, so not a lot of teaching time, instead they tended to gather as a group of friends and work together, more as downtime from their more academic subjects, than as homework.
An extra-ordinary number went on to study some form of EEE. DD intercalated in Bio-medical Engineering where having the electronics A level both helped her get on the course and gave her real advantage when she started. DS went on to build his own computer(s), which has given him more practical experience. His most entrepreneurial friend made some money by putting his course work into small scale production.
Given there is an AS, and coursework, the exam pile up was not too bad. The actual science was pretty straightforward for those likely to gain top grades in FM or physics. What proved to be very useful was the requirement to do some coding, experience DC were able to use at University.