In Minecraft 1.12 & lower, you can create massive villages in Superflat by setting the distance to 9 & the size to 100 in presets. In 1.13 & higher, no amount of messing with the size & distance will get the villages anywhere near close enough.

Additionally, trying to create a super village on Superflat with a snow top layer & dark oak saplings in either version will fail to generate & crash.

In a very old Reddit post for Beta 1.4.2, Dinnerbone explained why the minimum distance was switched from 0 to 9. But in latter versions, the village generation was not as close-compact as his screencaps.  

https://www.reddit.com/r/Minecraft/comments/12lzxj/i_buffed_the_village_size_to_almost_infinite/

The village distance on the same settings increased heavily between Beta & 1.12 & then exponentially by 1.13.

Superflat generators like the one below, last updated in April 2018 according to Archive.org, give out codes for different versions of Minecraft. Either the preset code for villages has changed enough from 1.13 snapshot 18w14b to make the generator partly useless, or the 1.13+ villages are just loading further apart.

http://minecraft.tools/en/flat.php

I have also tried with without the layer of jungle saplings & get the same results.

This is potentially a side effect of MC-140238, where villages don't seem to spawn at all in Superflat, because I had to tweak the size & distance of villages just to get them to appear as close as they were, & on 9/10 tries in failing to create a supervillage on 1.13 & 1.13.2, no village appeared whatsover.