Do you mean specifically n digits, like "what are my chances of finding specifically a 48 digit factor if I run this curve?", or do you mean less than or equal to n digits?
Note your odds depend more on the amount of previous ECM done than they do on the specific bounds of the curve you plan to run, unless your planned bound is vastly larger than previous ECM efforts.
For instance, running a single curve at B1 = 3e6 has a nice chance to find a 25 digit factor, if no ECM has previously been run; but if a t35 has already been completed that same curve's chance to find a 25 digit factor is nil.
