Never.
Well, almost never. There are a few cases where a new catalog is in the cards. One is when adobe requires you to "update" your catalog in order to use a new version. But, I don't think that is what you are talking about.
If you have a masssive number of images in your catalog (let's say over half a million) and you almost never access those over, say, 10 years old, you may wish to split out the old images into an "archive" catalog. Some people do this when their catalog size on disk gets bigger than they like.
I've also seen people get to a point where they want to segregate images for some reason. For example, professional images from personal images (e.g. for a wedding photographer). another example is someone collecting scanned historical family images from many family members and wish to keep those seperate from their own photography.
Another example is where a couple were using a single catalog but then as they both got more and more into photography it became cumbersome for them both to not be able to work in LrC at the same time. So this would be a valid case for seperate catalog for each of them (of course that would make it difficult to put together a set of images for a trip from both of their cameras)
I'm sure there are other valid use cases for a second catalog but if we're just talking about your own images, in non stratospheric numbers, the answer is ONE CATALOG.