Keep one master catalog with all your images.
Keep your existing folder and naming structure because it works for you.
Do not make separate catalogs because to find anything you have to open them one at a time.
The LR Importing process only records where your files are. You can optionally use LR to copy from your camera/card to a variety of folder structures. Once done, you can make folders to your liking, move and even rename everything.
With one single master catalog and your existing file structure that you are happy with, you gain the ability to search by a lot of existing metadata. Then you can add collections and keywords as you see opportunities.
Tim Grey is one of the dozen well know on line teachers. He keeps something like 300,000 images in his master catalog. Try watching one of his videos: https://www.youtube.com/watch?v=wm6OroP6CAUhttp://
Good luck and have fun!
Should I make one huge master catalog with over 12,000 files and then make many image collections from that?
One catalog is the way to go. That way, you don't have to search for the desired catalog. You only have to search for your photos.
Use keywords and other metadata (captions and titles), not collections, as your primary organizing tool. Keywords (and captions and titles) can stay with the photos outside of Lightroom, while collections cannot.
Right now I upload all my images and sort them filename and date. A typical folder/filename pathway might be Photos/2016-02-20 Khadijah Event MC/Khadijah MC event Bendi 2016-02-20 13.57.35.jpg. I upload from camera or whatever to a central upload folder and then use Bridge to sort and rename and move the files to a new folder in my central photos repository with folders and subfolders.
How the heck do I incorporate that system into Lightroom? It takes forever to access a central folder with over 12,000 images in it.
Keep the existing photos in their existing folders. Which means that when you import the photos into Lightroom, use the ADD option.
Future photos should go into folders that are identified by capture date, and the additional information such as what event or who the people were (or any other relevant identifying information) go into the keyword and/or caption field.
And make sure to move the folders around from within Lr, so that it knows where they are. Bridge will automatically see those images since it browses folders. Lr has similar renaming options to Bridge, and keywording too, although sometimes some metadata entry is still easier in Bridge. Both work well together in that regard, as long as you remember that Lr has to know where the files are, and has to read from the files if you write changes to them in Bridge (like when you add a keyword in Bridge).