Well, ultimately the main cast does have the biggest share of content? Sure, the last years focussed on a ton of other characters, but none of them comes close to having the amount of content as for example, Roxxy, Debbie, Jenny or Eve. Some complained how many scenes Odette got in recent times, yet she's still quite a bit behind those I mentioned.
When it comes to the characters, Sploosh released a tier list some time ago, so we kinda know what's going on in their minds:
Looking at that list and the number of scenes we've got, I'd say that primary characters will get 10+ scenes, the secondary ones something between 4 - 8 and the teritiary ones 1-4 or so.
Edit: Just to add, the huge number of secondary and tertiary characters, at least in my view, makes the game feel quite a bit more alive. In most other games you've got your cast and no much else. In SS on the other hand there are all of these "normal" characters around (teachers, shop keepers, nurses, nuns, police officers, cleaning ladies, etc.) who are available as well. As it it really feels like MC is actually, really fucking a whole town and not just "his girls"