You need a reference manager the day you know you saved something and cannot find it — not before. Under a few thousand images, a folder tree with sensible names and a handful of Finder tags answers almost every question you'll actually ask it. Past that point, the minutes spent trying to remember which folder you used start costing more than any tool would.
The symptom, not the number
Nobody hits a wall at exactly ten thousand files. What actually happens is a handful of specific moments start repeating: you can picture the image and still can't find it, a folder named "references" from two years ago could mean three different projects, or you've quietly stopped saving things because filing them feels like more work than it's worth. Any one of those happening occasionally is normal. All three happening weekly is the actual signal, and it has nothing to do with a file count.
What changes at scale
A folder gives every file exactly one home, which is fine as long as you remember the decision you made when you put it there. The failure mode isn't the folder structure — it's the untagged, unfiled pile that accumulates alongside it: the batch saved in a hurry, the folder somebody sent you, the three hundred stills pulled from a film one evening and never sorted. Scrolling still works on a library that fits on one screen. It stops working the moment the pile is bigger than your memory of what's in it.
What a reference manager actually adds
The honest list is short: search across facts that are already true of a file — its type, dimensions, rating, tags, notes, when it was added — without anyone having labelled it first; a way to save that search so it stays current as the library grows; and, in Coii Ref's case, a canvas for arranging a set of images without moving or duplicating them. None of that replaces a folder structure. It sits on top of one.
When the honest answer is "not yet"
If your library still fits on one screen, if a handful of Finder tags covers how you actually think about the work, or if you can find what you're looking for in under a minute more often than not, a reference manager is a solution to a problem you don't have yet. The tags-versus-folders question is worth reading before spending anything — most of what a paid tool adds is also available for free, just with a lower ceiling.
Coii Ref's specific answer
Coii Ref indexes the folders you already have — nothing is copied, moved or renamed — and reads what's already true of each file: its dimensions, EXIF and colour. That means the untagged pile above is searchable on day one, before you've tagged a single thing, which is different from tools like Eagle that bring files into a library on import rather than reading them where they sit. What a reference manager is built to do covers the fuller feature list, and the one-time-purchase shape of this one specifically if price is part of the decision.