civilization-6
As far as I know, borders can grow when a certain amount of culture is accumulated in that city.
But how does purchasing tiles affect this cultural accumulation? Do the accumulated culture become nullified and start over or are they kept and carried over to the next new tile?
Tiles up to three away from the city center can be selected to be worked.
Additionally, the number of citizens is the limiting factor for worked tiles. Let's assume a newly settled city has one citizen. Even if you buy tiles for that city, only one tile will be worked since the city only has one citizen.
As of the "Civilization VI Fall 2016 Update", this is now viewable by default.
At launch, in the base game, there was no way to see when or where the next border expansion will take place.
Best Answer
I was able to test this myself and confirm that a few Reddit threads, including this one, match my observations.
If you buy a tile, it does not reset the culture progress towards acquiring a new tile. However, it does increase the amount of culture required to claim the next tile, but all progress is retained.
Comment from the thread: