So, who are they?
Only nine AI are named in the single-player Career mode of Blur, but from alpha and pre-beta gameplay it's possible to note that Blur originally had a much longer singleplayer career and thus, there were originally far more named AI players. The actual amount of AI included in the game is unknown but there are 16 confirmable named drivers, all of whom do exhibit their own unique driving styles and can be differentiated by their driving skill and weapon usage.
Here is the list of the AI, taken from a 4-player private session scoreboard. As there are 4 AI assigned for 1 player in an online session, this means that there can only be a maximum of 16 registered AI (when 4 players are connected). There are no extra AI so therefore no possibility of there being different entrants.
Some of these characters are interesting to note, for example:
The single-player character is in fact Casimiro, the guy wearing the sunglasses and having black hair and a beard.
Multiplayer races assign player models randomly from a small list. Assignable models are as follows: (incomplete, I will add them as I figure them out)
NO-1 - the large guy with the Afro;
Casimiro - the character portrayed in Single-player (initially the player character was in fact intended to be a teenager in a Hoodie);
Talon (Everyone knows who Talon is);
Morrisey (the man with the blue plaster on his forehead);
Maaz (the man with the turban/bandanna);
Ayumi or Harumi (it's one of the two);
Jun (the Japanese guy with the short ponytail);
I believe Laraya is playable but this is unconfirmed.
Enjoy this information, however pointless is may be.