See, back in Lightroom 1, it became fairly common knowledge that when your catalog grew to a certain size in photos (say 20,000) that it would slow down and you should create a new catalog so Lightroom would keep running quickly.
But that’s not the case anymore. Lightroom doesn’t have a photo limit that anyone has hit. Adobe doesn’t even recommend creating multiple catalogs anymore. So the first thing I tell people is to stick with one catalog. Keep it simple. I’ve got 70,000 photos in one of my catalogs and I don’t notice it to be significantly slower than a smaller catalog.
Now, does that mean you should never create multiple catalogs. Nope. It’s like many other things out there. Know what the rules are, then know how and when to break them. I know wedding photographers that create a new catalog each week for each wedding. Honestly, if I were a wedding photographer I’d probably do the same. I know people that create a catalog for their personal photos and one for their professional photos. Me personally, I don’t agree with that one. But hey, if it works for you then go for it.
My point is, if you have a good reason and are an advanced enough Lightroom user then creating multiple catalogs may be the way to go. But keep it mind, multiple catalogs is an “advanced” thing to do in Lightroom. You’re not going to find a lot of support for it, you’re going to complicate your workflow, and you’re not going to find a clear cut path on exactly how to make it work well.
But I think for most people, one catalog will work just fine. Load it up with as many photos as you’d like and don’t sweat it. It keeps things simple and, in my book, simple is usually good when it comes to managing our photos.