DS's prospective sixth form is unusual in that they still enter everyone for AS levels. They all take 4 ASs and then mostly drop one and take 3 A levels. It's useful to start off with 4 IMO as the ones they decide to keep can be different to their GCSE favourites. They also have all doing an EPQ with a weekly lesson dedicated to it.
At the selective school I work at all start with 4 (some really bright DC start with 5 and take a 6th in their own time!) and can drop one at the end of Y12 but some continue with 4 (or more.) No ASs anymore, though. They are encouraged to do an EPQ, but it's not insisted on. Over half do, though.
Some universities take EPQ into consideration and may include it in the offer. At one of my DS's choices (for a maths degree) he could have had a B in physics instead of an A if he got an A in his EPQ. I'm not sure if one grade down is worth the time necessary to get an A in an EPQ though! It did make him research and write though, which is a great life skill for uni and beyond. If he'd continued with 4 A levels he wouldn't have done the EPQ as well.
Universities may take 3x A or 2x A plus 2x B but again, is it worth the drop of one grade for the time taken to study an extra A level?
I think for FM it's a bit different. Maths, FM and say Physics only, is a bit of a narrow field of subjects, especially if you decide you're not that keen on maths after all! (Caveat, that's what DS1 took and is now happily studying maths at uni, but he also took computer science to AS)