I would say a good 12 or so quite easily of any of the larger varieties, leaving room for upper stocking, 15 or 16 of the smaller (C. Pygmaeus, C. Panda, etc)
If you wanted purely Corys, i would make sure theres a good few places to hide out, and plant out the tank so as to block a fair bit of the light, growing near the top, not too bright at the bottom, they dont appreciate bright light a great deal, some will hide a lot. You'll have a bottom heavy but very interesting to watch tank.