12, assuming the quilt shop actually cuts in a straight line.
I say that because on my last trip to my LQS I bought several snippets & several fat quarters. I had some snippets off my more than 1" -- which was fine on the one that gave me 11" in one direction, but not so great for the next 3 where it was 9-9 1/2" in both directions and one that was only 8 7/8". I thought perhaps there was something wrong with the fabric & it shrunk a TON in my washer even though I put it on cold handwash cycle & low heat, but sure enough, when I went back to pick up a few more snippets & measured them while I was still in the store, they were also coming up short. These are expensive repro fabrics, so there's really no excuse. After I finish my current quilt, I probably won't ever shop there again. There's just no excuse to have that same problem with so many cuts of your fabric from different fabric lines. Especially since they gave me this whole long lecture about how "precise" their fabric cuts are since they tear instead of cut. Clearly, it isn't working. The torn edges are at a diagonal -- 9" wide at one end and 10.5" at the other. I don't even know how they managed that. These are Andover & Moda fabrics.
If you're not getting twelve 12 3/4cm x 12 3/4cm squares out of your fat quarters, it might be time to check your LQS's cuts.