I haven’t counted my circs recently, but I’d estimate I have about 75.
(I know; it’s a sickness.) The collection has outgrown one of those Circular Solution holders, which are great for smaller quantities.
I recently bought a 3-ring binder at Staples with a 3" spine. Really fat. Then I bought a bunch of heavy-duty plastic bags with zip-lock type fasteners. These come 3 to a package and are punched for use in binders. Each size circ, regardless of length, goes into its own bag, which is marked with the number. And all the bags go in the binder. To find the right needle, I just flip through the “pages” until I get to the right size, and pick the length I want out of the bag. There are currently bags for sizes 0 and smaller through 13 and larger.
The “needle book” is bulky but it stands on a bookshelf like a dictionary and works very well. I’ve never felt so organized.
As for knowing which needle is what size, I keep a sizer in the pocket of the binder.