If you have more than one face-down development card in Settlers of Catan, do you have to keep them distinguished?
The rules, section 3d, say "Keep your development cards hidden (in your hand) until you play them, so your opponents can't anticipate your play."

In games I've played people tend to stack cards in front of themselves rather than completely conceal them. Even so, long as they don't play a card on the turn it was purchased, there is nothing stopping them from stacking/ordering cards in whatever way they choose.

So, yes, players can certainly mentally keep track of how many development cards someone may have picked up, there is nothing requiring them to be visible to other players after that point.