The Druid Circle of Dreams gives, at 14th level, the Walker in Dreams feature (XGtE, p. 23). I'm unsure about when it is used. It states, "when you finish a short rest" and "once you use this feature, you can't use it again until you finish a long rest".
The game allows for two or three short rests (not sure exactly the number) between long rests. Do you pick one short rest and use the feature immediately after you finish it? Do you get to use it any time after you finish the short rest?
Like you quoted, the timing for the ability is simply:
When you finish a short rest, you can cast one of the following spells
This means that each time you finish a short you can choose to use this feature (you don't need to plan ahead). However, once you choose to use it, you must wait until you finish a long rest before using it again. That's all there is to it.
As a small note, there is no hard limit on the number of short rests one can take between long rests. The DMG does state a typical number of rests:
Short Rests
In general, over the course of a full adventuring day, the party will likely need to take two short rests, about one-third and two-thirds of the way through the day.
But this is more of an observation and by no means a limitation. See this related Q/A that goes into more details on short rest limits.
