At least in S1 and S2 there were more than 1 episode lol. Small detail though, it was full 4 years, with few days of differences between releases. But if I remember correctly, there were some budget constrains, but it's fuzzy already. At least from S2 to S3 there was just a year of a difference, + 3-4 months, and -3 months to S3 part 2 from part 1.
Oh wait, I remember...there was lack of content for Season 2, since they aired S1 quite fast and it hyped us pretty well, and they had to make manga, to get the content for Season 2, and they produced enough of content that allowed for Season 3 to be released the year later as well.
But yea, at least per season there are 10 eps, meanwhile this one has episode per year or two...
Update: I just googled quickly about Drifters though, it seems there wasn't enough content for Season 2, but there will be, though as far as I understand the Season 2 will finish on 20tu episode, right now we have 15 episodes available, 3 of those as OVA's.
https://www.quora.com/Will-there-be-a-second-season-for-the-Drifters-anime-And-if-there-will-be-then-when
Basically I got my answer now and mind at ease...should've done that initially :D