I did 6 Highers in S5 - along with most of the rest of my class.
The norm nowadays is indeed just 5 Highers in a single sitting (in comparison to the English 3 A Levels). However, it is still possible to sit more - especially if you do more in S6. Ds will leave school with 6 (good) Highers and 2 AHs. He chose "only" to do 2 AHs and a crash Higher in S6 because he is also coaching - both at school and outside (as well as being academic, he is also sporty). One of his fellow pupils in S6 will be leaving with 6 Highers and 4 AHs (having sat one of the AHs last year, getting an A).
Ds' school allows pupils to sit 8 x Nat 5s if they are capable (plus in ds' case - and presumably most of his cohort - he also ended up getting a Nat 4 in RME, just from what he did in the core hours - and if he hadn't been doing a Nat 5 in PE, would've got a Nat 4 in that too from the core hours).
Those schools that only "allow" fewer Nat 5s will have studied a broader range to a greater depth for an extra year, so that it is easier either to crash a Higher (in S5 or S6) or to supplement more Nat 5s (although I prefer the way ds' school does it).
In England, English is split into English Language and English Literature, which accounts for one of the "extra" exams (in Scotland, language and literature are combined in the single exam).
I've been very pleased with the quality of ds' education - and I know he has an excellent foundation for Uni, wherever he ends up
(His preferred choice is notoriously late in sending out offers - but he already has an Unconditional from a highly respected Uni).
Scotland must be doing something right with the quality of its exams because, as I repeat, UCAS, even though it recently re-calibrated its points system, values the Scottish exams more highly than the equivalent English ones. 