Not quite the same application but from my experience with copper sheet, getting the cutouts to fit just right is a huge pain: What did you cut on your Shapeoko/ Nomad today? - #4445 by HeuristicBishop
The bits of sheet will catch on the grain on the wood and cause the metal to enter at an angle. That makes it catch more and bend which makes it catch more…so if you’re going to inset any more than the inlay of your material, I’d be hesitant to recommend sheet inlay.
That said, if your numbers will be roughly flush with the surface of the walnut (like most inlay I suppose) the inlay for numbers might be relatively quick and easy.
One benefit of this approach is that you may be able to punch the brass numbers from your sheet stock. That would give you very clean inserts even with a very thin sheet and should have good yield in terms of the raw brass.
A different but equally annoying approach that could look wild might be foiling. That’s a whole different beast with a bunch of different methods for application and protection/durability but it’s dirt cheap and can be very fast and repeatable.
The upside here is that it’s quick and easy to try your hand at it if you’ve got patience and time to spare…do one really well and see if it’s worth pursuing more efficient methods 
Ok lastly I have to mention it because I’ve never seen it done but it sounds super cool…you could apply (dust|burnish|paint|etc) a conductive material in your number pockets and simply coat it with a brushed metal plating application. Nickel and copper plating solutions are very easy and cheap to make yourself and can yield some impressive results both aesthetically and in terms of durability.
I’ve got some nickel and copper solution on hand and I’ll hopefully be getting some walnut tomorrow. If I try it out I’ll post some photos and let folks know how obnoxious the process is (or isn’t) 