Underneath all of the features and tools, Lightroom is, at its heart, a database program. One of the things that make Lightroom such a powerful and flexible application is its ability to help us find our images. As our collection of images grows it can become more and more difficult to locate that image of the puppy in the basket by the red flowers. But the keywords, metadata, collections, folders, and so on are all hooks Lightroom can use to search and find that puppy!
There is nothing in Lightroom to prevent you from creating more than one catalog to store your images. But should you? Let's take a look at the reasons you might decide on one approach versus the other.
The Single Catalog Approach
The idea behind maintaining only one catalog stems from Lightroom's extraordinary keywording and search features. Since, at its current version, Lightroom cannot search across multiple catalogs, it makes sense to have everything in one place. Otherwise you either have to remember which catalog the image of that puppy is in or open one catalog after the other, do the search, try again. Well, that's not much better than just having everything on the drive and relying on folder names to categorize images.
Prior to version 2.x, Lightroom seemed to choke when your catalog got over 10,000 images. But with the current version and 64-bit code reports are that catalogs of 100,000 and 150,000 images are doing just fine! How large a catalog you can maintain depends on your underlying equipment (processor speed, RAM, drive space, etc.). One catalog makes your organizational life easier.
The functionality of multiple catalogs can be replaced by Lightroom's Collections and Smart Collections. You can categorize your images, view them in their groups, and still have everything in one catalog.
The Multiple Catalog Approach
So why use multiple catalogs? There are valid reasons. Perhaps you want to keep your professional work separated from your family photos. Creating two catalogs in this case is perfectly valid.
Some wedding photographers will use one catalog per wedding. That keeps their workflow contained and helps prevent mixing up couples when presenting the slideshow or contact sheets. Here multiple catalogs can avoid embarrassing moments. And, if you have no intention of selling Bridal stock images, you may have no need to search through the different wedding catalogs.
Multiple catalogs will keep each individual catalog smaller and leaner. On older equipment this can be an answer to the performance issue.
Catalogs can serve as logical dividers if your photographic work spans several creative areas. For example, a catalog for your Landscapes, one for your commercial/product shots, another for studio work, and so on can help you keep those disciplines apart.
Keep in mind that if you go down the multiple catalog route you are giving up some of the search capabilities of Lightroom. However, if you have good reasons and it makes sense for your workflow then, by all means, separate your catalogs. There is no correct answer except the one that works for you.