Apart from hentai, oneshot and 4-koma, anything is welcome.

P.S: According to my experience, I would stand for the view point that: the fewer the characters, the more time the mangaka spend per character, and thus the more concentrated the story gets.

Problems can be anything but a problem.

What rises to the level of a "character" for you? A person appearing in the background of one page? A person with dialogue on a couple of pages? An actual named character the protagonist interacts with on multiple occasions?

Series like Little Forest and Tales of a Charcoal Burner focus mostly on the protagonist, the other characters are basically extras.

Same with Journey to the End of the World and God's Child, although in a much darker, more surreal fashion.

Tuesday, 9PM & Fantasy World Jun have essentially two characters.

In terms of longer series, Yokohama Kaidashi Kikou has relatively few characters for a 14 volume work.

Quote from hahhah42

What rises to the level of a "character" for you? A person appearing in the background of one page? A person with dialogue on a couple of pages? An actual named character the protagonist interacts with on multiple occasions?

Well, the subject of "types of characters in fiction" is not a clear one. People have opinions. As for me, I can't say there is a certain limit of presence or appearance a person have to pass in order to become a character. I would however say that every person and every animal is a character.
For instance, "A person appearing in the background of one page" would be a background character, "A person with dialogue on a couple of pages" would be a supporting character, "A actual named character the protagonist interacts with on multiple occasions" would be a main character and the protagonist would also be a main character.

That said, I thank you for the series you mention, The Journey to the End of the World in particular.

I thought that in general, episodic series like Yokohama Kaidashi Kikou would be more bound to have a considerable number of characters as they often introduce new characters every episode.

Problems can be anything but a problem.

Quote from gonubana

I thought that in general, episodic series like Yokohama Kaidashi Kikou would be more bound to have a considerable number of characters as they often introduce new characters every episode.

Not really. It has far fewer characters than most much shorter manga I've read. The setting is post-apocalyptic so the world's practically deserted.

As for recs:
Olimpos
Satougashi no Dangan wa Uchinukenai
Tekkonkinkreet
Eien no With
Conductor
Toukei Ibun
Akusaga
Tsuki to Mizu no Yoru
Paperweight Eye
Suiiki
KissWood
Hakka Haien no Shujin to Shitsuji
Akumu no Elevator
Shissou Holiday

Quote from calstine

As for recs:
Olimpos
Satougashi no Dangan wa Uchinukenai
Tekkonkinkreet
Eien no With
Conductor
Toukei Ibun
Akusaga
Tsuki to Mizu no Yoru
Paperweight ...

Without counting Suiiki, which I already read and enjoyed, the only one I could read past the first chapter was Satougashi no Dangan wa Uchinukenai (the pitfall was chapter 8 when the story was becoming a detective one).

I believe the detective tone in most of these stories was my insurmountable obstacle, but then, I said in the first post that anything was welcome. So, I can't really complain.
Therefore, I thank you for the effort and the 7 chapters that captivated me.

Problems can be anything but a problem.

Kuro Maybe, I tried counting the characters with names, and I had enough fingers.

Kami-sama ga Uso o Tsuku.
Bonnouji
My Girl (SAHARA Mizu)
Undercurrent
Watashitachi no Shiawase na Jikan
Natsu no Zenjitsu
The Friendly Winter

Fukuyadou Honpo has what I'd consider a small cast, but it isn't the smallest. Still, I'd recommend it because it does have a concentrated storyline and with some of the most fleshed-out characters I've seen. Great character growth and a personal all-time-fav.

Hope these work for you. Suiiki was great.

Quote from F_J

Kuro Maybe, I tried counting the characters with names, and I had enough fingers.

Supposing you meant Kuro (Somato), +1 if it had the fewest characters, -2 if it didn't.
From time to time it was reminding me of Binchou-tan.

Quote from VitaeVivae

Kami-sama ga Uso o Tsuku.
Bonnouji
My Girl (SAHARA Mizu)
Undercurrent
Watashitachi no Shiawase na Jikan
Natsu no Zenjitsu
The Friendly Winter

Fukuyadou Honpo has what I'd consider a small cast, but it isn't the smallest. Still, I'd recommend it because it does have a concentrated storyline and wi ...

This made me finish Kami-sama ga Uso o Tsuku which I had dropped.
Also, I already read Undercurrent and it was a delight.
So, thank you.

Problems can be anything but a problem.