I don't think there's any actual official rule to it, but I do feel there's obvious reasonable limits to what is and isn't as far as forum threads go for a story. Namely, they should serve a purpose unique from the other thread, but just about everything can probably be condensed into two or three threads for a thing, if you even need more than one. Though if you mean the thread tag thing for the stories on the main site, there doesn't seem to be an actually limit, which is kind of annoying because how insanely spammy authors get with putting every tag under the sun, even if they are tagging things that aren't actually involved and in a lot of cases they kind of just mean things that may happen as opposed to what the actual story contains. I really feel like there should be a push to limit how many tags someone is allowed to seriously spam for every little thing, myself, but I understand it's also not easy to condense everything of a big story into just a few keywords. At the same time, 30+ thread tags for each chapter is a little ridiculous and annoyingly clickbaity, in my mind.
If you refer to options of a single chapter, there should be basically no limitation. It depends on the length of the tags. I guess it would technically be possible to have 5,000 to 10,000 tags.
