I have 3 to keep things separate, photos the kids take with their cameras, my regular photos, and my studio work. I just don't want the others to show up in my general catalog.
The catalog files are small, my main has some 45k photos cataloged and is only 431.8 MB on disk. It'll never bog down your computer, it doesn't load the images along with the location info.
I can see wedding photogs having one for each wedding and then moving them to storage when done, maybe keeping portfolio keepers etc in an other catalog, but for general use there's no reason. And since you can't switch on the fly, LR always has to shut down, it's just a nuisance to deal with more IMO.
I thought about going by year, but even that really makes no sense and considering the xmas/NYE holidays, I'd end up with some photos of that here, some there. I guess one could switch at the end of some slow period, but why bother. If I'd move them off the computer into a closet or safe (like maybe wedding and event photogs do after a while?) I can see the point, but if you have all photos accessible all the time, keep them in one.
I don't know what experts suggest to have many catalogs, Martin Evening suggests one and I believe Kelby does too, though I don't have his books.
ETA: if you have a large catalog, you can optimize it (which I guess cleans things up neatly) right under the file menu. Back it up first just to be sure.
and this from Adobe:
How many photos can I have in a catalog?
There is no specific maximum number of photos you can store in a Lightroom catalog. If your catalog grows large and you notice Lightroom slowing, see Optimize Performance for suggestions on how to speed it up.
found here: http://helpx.adobe.com …atalog-faq-lightroom.html
, where that Optimize Performance leads to this link: http://helpx.adobe.com …erformance-lightroom.html