I would strongly urge you to use only one catalog as my understanding is that Lightroom can only handle one catalog at a time. If you are working with images in one catalog (call it Catalog A) and want to access images in another catalog (call it Catalog B), then you have to close Catalog A and open Catalog B. Similarly, if you are working in Catalog B and want to access and image in Catalog A, then you have to close Catalog A and open Catalog B.
A lot will depend on the type of photography you do as I know some wedding photographers use a catalog for each wedding, but if you are just photographing images on a general basis, as I do, then one catalog will provide you with the least amount of confusion and grief. I have over 70,000 images in my catalog and Lightroom handles this load very efficiently. I have heard of a one professional who has over 250K images in a catalog with no problems.
I would encourage you do two things as you start out: 1) make very liberal use of keywords for each of your images and I do mean liberal. This will give you a broad ability to find any image in your catalog in a few seconds, even as your catalog gets bigger; 2) learn to use Collections, particularly Smart Collections. For example, I do a lot of landscape photography, often more for the clouds then the landscape itself, so I have a Smart Collection called "Clouds". This Smart Collection is set up so that every image that I take that has the keyword "cloud", or "clouds" attached to it, is included in that Smart Collection, so if I want to look at my cloud images, I just have to go the Smart Collection and I don't have to do a search of my images every time I want to find a cloud image. Because I use a Smart Collection for this topic, every image that enters my catalog that has either key word attached to it is automatically included in the Collection.
There are lots of YouTube videos on both Key-wording and Smart Collections on the web, however, should you run into problems, post a question here and someone will provide you with an answer.
Hope this helps.