There is no such limit, and in fact some users in this and other forums report catalogs of over 1/2 million images running well.
The idea of creating multiple catalogs because you don't want the catalog file size to get too big is such a bad idea that you should never think of it again.
Currently there is a minor problem with ZIP compatibility with catalog backups when your catalog is bigger than 4 GB.
Adobe is currently working on that problem and there are workarounds available.
All that said, as DJ points out, you are better off with just one catalog.
Thank you sir!