I assume by S series you mean the side stories which start off with the reincarnated babies. Since several events changed over time, this summary skips their childhoods, but will not go into details about the major story events that happened as they are teenagers to avoid major spoilers.
For manga readers at least, this will be full of spoilers from the light/web novel. The spoiler tags only hold very minor spoilers except the last few. If you watched several anime episodes or read several volumes of the light novel they are not spoilers.
4 of them
died while young and are not important to the story.
2 of them
were born in the major kingdom Analeit. The first being Shun a prince and younger brother of the one with the hero title, and the other Katia, the boy (Kanata) who reincarnated as a girl.
Another one was
Hugo, born as the next in line for the strongest swordsman, a Prince of the Renxandt Empire
And all 3 above were sent to the same school, along with the Teacher (Filimos/Oka), who
was born as an elf, the child of the leader of the elves.
One more also attended the school
Yuri who was orphaned to the church and became a potential Saint Candidate.
A change from the Web Novel to the Light Novel (which the Anime and Manga are based off of) had 1 more attend the school
Feirune, the girl reincarnated as an Earth Wyrm and who is blending in as Shuns familiar.
born in a small village and became adventurers together after it was destroyed and everyone else killed. Eventually as teenagers they were collected by the elves and joined the 11 below
11 (12 in the web novel) were
picked up, bought, or kidnapped by the elves and raised in their village, isolated from the world.
The last 3 here are encroaching in real spoiler territory as their backstories play an important role in the plot and the last 2 are not even revealed yet at all after 20 Anime episodes.
born in a town as a progenitor vampire, to the local Lord of the land.
1 more was
Found by the Church and became one of its elite operatives.
The last one was
Was born as a goblin.
Somewhat major spoiler on what happened to 2 of those 3
Sophia and the Goblin went through some major spoiler events and eventually joined the Demons as elite operatives
That is all 26 reincarnations in their early life excluding our Protagonist.