There is zero online players as multiplayer isn't released yet. Noone knows how popular it will become, and that of course depends on how multiplayer shapes up.
The game is worth to buy anyways imo. And I'm confident that Kunos makes everything to get multiplayer to work properly.
@leffty in the moment connect with the RaceDepartment group on steam and this AC steam group and you should be fine, so that whenever multiplayer option is released you will find experienced and very good people against whom you can race.