Memory is your only limit right now. There isn’t any caching or dynamic loading for the navigation graphs, but that said they are usually pretty small (depends on complexity). Pathfinding also starts to eat up memory too, as it goes across the graph (and as the number of AI increase). We attempt to reuse and in general be smart about what we’re doing, but it could cause problems with significant size.
What you may want to do is a test your memory use of one of these graphs. Check the profiler before and after you generate it and see what kind of increase you get and you should be able to figure out if it’ll be an issue.
We do have some optimizations in the next version to help some more with memory, so if you find you are having issues we can see about working around it.