I would think something around the five mark, but that again.... I think one person you need to speak to is the person who runs this at the White House, who is in charge of all the technical back end. We got the idea of IPs from other jurisdictions, so I think they could give you a range that would be appropriate.
The other thing too is it's not just looking at the IP address. It's that the person has to actually answer an e-mail to say “Yes, that's my signature.” If you think about the steps, if somebody wanted to fake 100,000 signatures, they'd have to basically get 100,000 computers or around that number, maybe 75,000.