Per ESPN:
There are also some players who are wearing uniforms without numbers. (The numbers will, presumably, be photoshopped in later.)
One of them is Mario Chalmers, as you can see in the photo above. I asked him why he didn't have a number yet, and he said "I'm trying to get number thirteen." Alexander Johnson has that right now. Chalmers says he's working on it.
Its weird because Mario apparently doesn't want #15.