One of the reasons we store the email addresses is to stop people signing the same petition more than once. That's one of the things the system does automatically. We don't stop an individual signing a number of petitions. There are no restrictions there.
As to the IP addresses, I'm told by my technical team that in some large organizations, if they share a Wi-Fi connection it would have the same IP address, so they don't limit by IP address but they do look out for unusual patterns. I don't think there's a threshold as such, but we don't restrict by IP address because of that problem with Wi-Fi connections.