About 36 million, including some duplicate email addresses.
No. It is possible someone accidentally or deliberately registered with the wrong email address. Determining the likelihood of this is left to you.
You'd be amazed how many people apparently don't know their own email addresses, so be skeptical about the results, especially if the person's name is common.
You don't, but I'm the same person who made the Adobe leaked password checker if that lends me any credibility.