There have been over 22 thousand tanks created in TankPit (22,219 last I checked, to be exact), according to the profile ID system. This includes previous tanks which have been deleted. This is not the number of accounts, but the number of tanks.
If you find an ID number that shows "Tank not found." when you try to view the profile, that is a deleted tank's profile.
All tanks from id=1 to id=499 are no longer around (some deleted, I guess, and some never existing in the first place).
TankPit HQ is the oldest current tank, with id=500. Eva Is Back is second oldest with id=501.
red-1 (with a gold trophy) is the oldest deleted tank with id=1.
id=1 through id=9 are red-1 through red-9. 10 - 18 are purple-*. 19 - 27 are blue-*. And 28 - 36 are orange-*.
It appears that the first tanks to acquire trophies were red-1 (gold), red-2 (silver), and purple-1 (bronze).
id=37 through id=499 are "None." just blank, so it appears that there were no tanks created for those numbers; they were just put there to bring the number to 500 for the real players (apparently).
The bulletin board started January 4, 2012 (see here).
Bulletin board is empty (went down) from May 6, 2012 to October 24, 2012.
The "Created:" section on the profile only shows back to October 2012, even if the account was created before this.
The "Bulletin Board Posts" section of the profile only shows posts AFTER October 24, 2012.