I think that the first two replies missed the question here.
Any photo imported into Lightroom (any version) is put into in a 'catalog'....because a catalog is the file that LR saves to. I would think that most beginner LR users just use one Catalog.
Whether you use one or many catalogs, you should still have a sound method of organizing the image files (folders etc.)
The benefit of using one catalog is that you can easily search for any photos that you have imported. You can use the filters and keywords to find what you want or create groups of similar images etc.
The problem with one catalog is that it can get bogged down with a lot of photos. And the backup files can get pretty big as well. And really, if you have no reason to ever view/edit certain photos at the same time, there is no reasonu to have them in the same catalog.
For example, you may never want/need to view your client photos along side your personal photos. In that case, you could put them in separate catalogs.
Then you have to decide if you want to put all your clients into a catalog or create a new catalog for each client/job.
I know some photographers that put each wedding into a new catalog. This keeps things tightly organized, but the down side is that if they want to search 'bride by a lake', they might have to open several catalogs and search each one.
Going back to where you put the images...that is also important, but it's a separate issue from LR catalogs. LR can create a catalog from files that are saved anywhere on your system, so it technically doesn't matter how or where you put them...but it's still a very good idea to have a strategy and to stick to it.
I think that most people organize their photos with folder by date. You may want to have your client images completely separate from your personal files, but that's up to you.
What I do, is to have different locations for personal photos, weddings and portraits.
Also, you should have a plan for backing up your images.