Based on the erotic game by Marmalade.
What are some good anime series that don't have too many episodes?
Most of the other answers have some great ones too, but I’m just going to add some of my favorites that they did not list:
One Punch Man - 12 Episodes - Comedy, Action
Boku Dake Ga Inai Machi(Erased) - 12 Episodes - Mystery, Time Travel
Noragami - 12 Episodes - Comedy, Action
Hai to Gensou no Grimgar - 12 Episodes - Action, Fantasy
Deadman Wonderland - 12 Episodes - Horror(Still great though)
Rokka no Yuusha(Rokka: Braves of Six Flowers) - 12 Episodes - Action, Mystery, Fantasy
Tokyo Ghoul - 12 Episodes - Action, Monsters, White Haired Ninja Demon
RWBY - 16 Episodes - Action, Fantasy, Rooster Teeth
===========Here are some others that are a little longer but still great==========
Sword Art Online - 24 Episodes
Attack on Titan - 24 Episodes
Death Note - 37 Episode(The last episode sucks)
Akame ga Kill - 24 Episodes
Parasyte - 25 Episodes
I hope this helped and it wasn’t too overwhelming.
Check out the other responses for others.