Many times I come across this question - Should we use single Catalog or multiple. It's not very simple to answer, because everyone has different workflows and use-cases. Some folks prefer mutiple catalogs and there is another set of folks who recommend single catalog. All of them have right reasons for their respective decision. Recently we had the same discussion on 'Indian Lightroomers' facebook-group. Here I have added these screenshots from same discussion and trying to summarize various reasons for having single catalog or multiple.
As performance of Lightroom is gradually increasing single catalog can handle lot of images which makes it easy to move to single catalog. Perceived performance used to be one of the reasons for having multiple catalogs. Now performance of Lightroom6 is brilliant. Currently I am using single catalog with 90,000+ raw images and it works smoothly.
Unawareness of Collections is another reason which leads to the decision of having multiple catalogs. When folks know about the power of collection, they start searching for option to import from other Lightroom catalogs.
Let's look at the benefits of single catalog -
1. All of your images are at one place and easy to manage.
2. If you want to search/find some specific photographs, you need not to move between the catalogs. At any point of time, only one catalog can be opened in Lightroom and if you have single catalog, you need not to worry about such scenario.
3. No scope of confusion :)
Why to go for multiple catalogs?
If you click different types of photographs and want to keep them separately, then you can choose to have multiple catalogs. E.g. - If you click Weddings, Travel and personal events, then you can have 3 catalogs for each.