At the time of writing, nearly a million have signed the petition to revoke article 50 on the parliament site, despite numerous crashes and slowness of the confirmatory email. Worth joining in, not sure what database parliament it is using to confirm ID, most departments having access to both the driving licence/passport database and NI database, often cross-check them if you are trying to access government services. Requires name, email and postcode, so work that one out for yourselves. Find the petition here.
Over two million have signed the petition at the time of this update but it took about six hours for the confirmatory email to turn up and then had to wait until late at night until the site worked well enough to take the link... so probably a lot more than two million have tried to sign up. BTW I am not on the electoral role so they are not using that to confirm ID.