Board Thread:General Discussion/@comment-31832165-20170517234623/@comment-31497829-20170518001224

It isn't limited by time, but open slots. You can't summon to a location in anagram curse (blue cba circle) mode, novice level, or with another anomaly on it.

The number you can summon MIGHT be limited by player's level, or how many are active at the time. I suspect the latter, since I've been given the limit message while I still had open locations, and I was in my 80s.