A lot of people have asked so here are the Printable Muffin #s. This is the Link where I got the idea.
I printed the #s on cardstock and used a big circle punch to punch them out. There's a video and then instructions on the link. Basically you cut out twenty four 2 1/4" squares out of cardstock and then use a corner rounder. They did circles and squares, I simplified to just squares. Then you glue the # circle on top and add a couple strips of adhesive magnet on the back. (Comes in the roll at Walmart). Then embellish.