I guess I really might have to read the guide but I am afraid I will be spoiled a lot more than just knowing the number of sex scenes of the girls.
I haven't read the guide, so I can't comment on what would be spoiled, but to answer the rest of what you're asking, there are a few things to note.
I'll start by saying that I play games for the stories, so the sex scene component isn't important to me. There's a decent amount of per-character storytelling, some of it is now well interwoven with other characters, and everything is well gated, so progression is smooth and things don't come out of order. The main story is still pretty light, but when you factor in all of the things going on with the girls, there's enough story to get a good read.
In terms of sex scenes, if that's why you're here, there are a lot. Many of them can be repeated through random events, but some don't repeat. There's no gallery that I'm aware of, so once those are done, you'd either need to load a save, or start a new run to see them again. Many of the one-shot scenes do share renders with later scenes (so they kind of repeat), but I don't know if that's going to be a long term goal. In terms of people like Jennifer, some of her scenes don't repeat at all yet; Amy, without spoiling it, has an event where the renders are re-used after a main story event is done. It's possible that we'll get everything 'sort of repeating' at some point, but, as I said, I'm not really here for the sex scenes, so other than them being needed for quests, I typically didn't push myself to repeat them.
You can skip a lot of repeatable sex scenes by turning down invitations. If you turn down, rather than don't show up, there's no penalty (that I'm aware of at least). So, if you're like me, and are more interested in progressing a story, or progressing the story of a given girl, then you can typically turn them down. But if you get a message inviting you to do something, check your quest log before you turn it down, because it may be that you're being invited to progress a quest!
So there is no clear answer as to who you should choose at the start? Because some girls are needed to trigger events even though they (the needed girls) may not have sex scenes themselves?
It doesn't matter who starts on the island. There are a limited number of slots (unless you use a console command to change it), meaning that only a certain number of girls are there at once, but, other than a few staff members who turn up over time (or in response to events being finished), you have access to the pool and can change them out (the staff stay and you can't send them away; they don't occupy slots). You can talk to reception, and use Company Favour/Credit to invite or remove girls. You need to send Yvette away as part of her main story/quest, and then bring her back, so even if you change the maximum resident value, you'll be using it.
You do get a unique scene from the 'girl on the plane', and a second unique scene. I don't think there's any lore there of value there, nor story related content. It does make that person easier to start with, and progress through their main story/quest as a result. You can only have one girl in that position per run, so if those scenes really matter to you, you'll have to start a bunch of runs.
Or are you saying to have access to the most sex scenes you should have Yvette, Jessica, Alice and Amy in your run?
(Even though you do not have access to the gym shower because Faye might not there?)
The game progression is very well done, and it's heavily gated. But other than that, it doesn't matter who you start with, because everyone is always available (you just may or may not have them on the island). This can be a bit of a pain at times, but it's not too bad.
You need to complete certain quests with certain girls to progress through certain areas (the sauna and access to Sarah come to mind). You can either mod the total resident count (which makes some scenes really hard to get) or you can invite/return as needed. If you don't like a girl, you can send her away. Staff stay on the island, but generally don't block 'need to be alone' events (they'll generally 'step away' as part of the event, or before it starts, after you attempt to do it).
If you get stuck, you'll either need to struggle through working out what's next, or use the guide. I didn't need the guide to work it out, but if you're not paying attention to what people say to you, the quest log may not be enough.
The game uses random generation in a lot of places, so saves and rollbacks can be good (I don't know if you need unren for it). If you want to rollback at all, make sure you don't use URM, as it prevents it. Because it's renpy, I don't think the random number generator changes on rollback, but loading does change it. Chances are it won't matter most of the time, but some low odds events are a huge pain. You can use the console to force certain events (e.g. removing the shower sign on the door after you see it, and/or forcing success for certain rolls).