As your career progresses and you accumulate more and more images, you will be more likely to start breaking off into multiple Lightroom catalogs. Eventually, however, you might need to recombine catalogs. This excellent video will show you how to combine multiple Lightroom catalogs while ensuring that everything stays the way it should.
Coming to you from Anthony Morganti, this excellent video will show you everything you need to know about combining multiple catalogs in Lightroom. This can be a bit of a nerve-wracking procedure, as you are moving around what often amounts to years of work. I used to prefer to keep everything in one catalog just because it made it easier to work on things like my website and because I felt more secure knowing all my images were in one place, but as my catalog started pushing into the territory of multiple terabytes, I had to move it to larger and slower external hard drives. I eventually settled on keeping my current year's work on a solid state drive and merging that catalog with my archival catalog on the larger external drive at the end of every year. Check out the video above for the full rundown from Morganti.