I need to show only the numeric reCaptha instead of character strings. Is it possible? If yes, please give the procedure.
Like Google's Street view
It is not possible at the moment. Your only options for customisation are basically just skins/themes.
If such an option were available, it would also not be in the options passed through on the client side as a bot could just request it as an easier alternative. Even the plugins only generate javascript to display the widget.
So the option would have to be server side, associated with your key and there are no options to do this when you create your key.
As mentioned in the Google blog, the numbers show up when it has decided you are likely human, so it gives you numbers to make it easier.