It's hard enough to manage thousands of pictures in one place - why fragment control of your picture collection and spread it across a number of catalogues? There are no cross-catalogue features for searching, and many other inefficiencies that are introduced.
LR can take tens or hundreds of thousands of pictures. The only thing that's slower is backing up a big catalogue.
Good reasons for a separate catalogue would include unusual privacy concerns - eg you shoot nudes that you don't want the kids to see.
There's a lot that can be said on this issue, and the debate gets heated sometimes ... but I strongly recommend one catalog.
In my opinion, the more photos you have, the more important it is to keep all of your organization in one catalog. Anything else results in confusion, where you don't remember what catalog has the photo of interest; or it results in barriers, you want one photo from Catalog A and a couple of photos from Catalog B and a few more from Catalog Q, and there's no easy way to put them all together.
I do point out that I agree with John, and I add another exception to his list: if you have COMPLETELY non-overlapping subject matter (like personal photos and business photos) and there's no chance of you confusing which catalog your Christmas family photos are in, then go ahead and separate business from work (but even so, there are drawbacks, if you're not careful, you could accidentally import personal photos into the business catalog and vice versa)
Thank you both! I was leaning towards what you have said, just wanted someone else to tell me. I appreciate your input and will keep one catalog and not really worry about it moving forward. Just need to figure out the best way to search the key words.