The common practice seems to be to add it below. See e.g. the comment boxes on any Stackexchange site or Twitter.
The important part IMHO is that the number is visible to the user when the information is relevant. E.g. if your input field is big and goes below the fold, but you want to give the user feedback to add at least 15 characters (like Stackexchange does) then it's best placed at the top.
Contrary, if you want to display the maximum amount of allowed characters then there's no use to display it at the top if there's a chance that the user cannot see it anymore when she has entered the maximum amount. This is especially critical on small screens.
As an additional feedback it might be a good idea to change the font color, e.g. to red, as soon as the user hits the limit (again, Twitter does so). The change in color is a clear signal for the user that something's wrong.