Incomplete numbered groups.

[u/macho_boy-420]

1. The Self Referential Six (Rick and Morty): excluding and Imprisoning Rhett from the group for supposedly being very dangerous.

2. Uno, Dos and Cuatro (Ferdinand): they don't want to talk about Tres.

[u/Virghia]

The opposite: The Hitchhiker's Guide is a "trilogy of six books" (The final book was written by Eoin Colfer after Douglas Adams' death)

[u/HelperRaven → u/Virghia]

I’ll do you one better. The original Marvel Transformers comic ended at #80, with the cheeky tag line “80 in a 4 issue series!”