Well, this answer is a bit wrong.
Currently, there is 28 girls with H-scenes.
-6 are the story girls. You can have them from the silver gacha only (they don't drop in premium gacha), in N/R/SR+ version. They are the most common girls and should ALWAYS be used as fodder. They will be UR very easily.
-Then, there is 13 others girls availables in the silver gacha. They come in R only (in the silver gacha, i mean). They are also available in premium gacha, and some of them in the exclusive gacha (more expensive). They will still be UR easily after a bit of time.
-Then, you have another 6 girls only available in premium gacha (or exclusive gacha) They come in R/SR/SSR+. The game grants some premium gacha draws, so you can be lucky (I got 2 SR of Charlotte, so I was able to SSR her).
-Then, you have 2 girls only available in the exclusive gacha (the expensive one). They will be available in premium gacha later.
-And finally, you have the event girl(s). For now, there is only one.
If you want to know the repartition i have, to give you an idea, I made an excel with my units, and for now:
Story girls: 7N/ 11R/ 8SR/ 3SSR
Non premium+ only girls: 13R/ 5SR/ 2SSR
Premium only girl: 1SSR.
Obviously, since I used only story girls as fodder, you can imagine the discrepancy in droprate. Still, you can manage to get a fair amount of girls.
Very easily. Actually, you can finish the hardest mission right now with 2 SSR lvl 60+ and the entire team composed of lvl 1 units.