We had a whole spew of spam bots register for about 6 or 7 months. Something like 20 a day. Eventually I figured out a better spam filter / captcha, and the spam registrations dropped to 0.
Then about a month later I went through a deleted any members who never posted. Some were legit members that, well, had never posted, but most were spam bots. That dropped the legit user count down to about 150.
So you might be the 2000th registered user, but 1800 of the previous registrations were just spammers.