Theoretical or practical library size limit?
Besides the storage sizes in the various paid plans, I couldn't find any information on the size limit that a zotero library can reasonably handle. I was curious as my Zotero appears to have been slowing down/crashing more as my library grows. I do have the unlimited plan.
Generally, and especially on Windows, Zotero 7 will work much better with large libraries. If you've been seeing crashes on Zotero 6 on Windows, these were very likely out of memory crashes that are much less of an issue with Z7 (which is a 64-bit app by default) Even with Zotero 6, though, libraries with ~20k items are the norm, and even libraries with ~100k items aren't unheard of, though you should expect some performance issues.
We know of libraries with up to 500k items, but that's getting iffy, performance-wise.
All of these limits are practical and also depend on the hardware used, of course. There are not theoretical limits imposed on library size.
https://www.zotero.org/support/beta_builds
Biggest concern before updating are incompatible add-ons (Zutilo and ZotFile, most prominently)