I have 3, one for all my regular photos, that's day to day family, food, landscape, etc etc, one for my studio sessions (mostly nudes) and one for my kids and their cameras. I did start out with new ones for each year when LR was new, but that really didn't make things easier, made it harder to find things.
But I like to keep the studio work separate - fine art nudes don't really work that well next to the kids in the park, and I keep what my kids shoot in an other catalog just so I don't have to wade through their photos too.
Kind of a similar approach I use with credit cards, I have one family card for everything, one for only my own stuff (amazon, gear, etc), makes it easy to track my expenses and since both always get paid in full I get free accounting support from the bank 
I thought about having them for each vacation etc etc, but that's really not practical at all, much easier to keyword all photos from particular events and keep it all in one catalog. That's also what just about every book I have about LR suggests and it makes sense to me, especially since you can't quickly switch but basically have to shut down LR to switch. I have no patience for that.