well the amount of users is going to directly reflect how much time you put in to trying to find new members (at first). each forum is different and thus its hard to tell how many users you should have after one month or so.. but remember its not about how many users you have... its about how many are active.. you can have 50,000 users... and none of them be active... than it really don't matter if you have 50k or just 5.
I think you are doing ok, but I would put the general forum at the bottom, its a car forum, put the car forums on top, where people can see it.. not the new members intro, when someone comes to your forum.. do you think they will wanna seee....a) a whole bunch of people talking about how they found your site.. or B) a car forum talking about...well cars?
other thing is you have way to many forums, start out with a few forums and work your way out when there is a need for them..... you don't have that many people posting and thus you don't need that many forums, cut down now.. add them back later when your site is boomin'.
Ok, yeah I didn't mean to go on and no about your site but over all its good, congrats on getting those active users... keep with it and try and make it a good success.
EDIT: one last thing... get a host of your own.. free forums are bad if you are serious about your sites.. there is a lot of people/companys that will give you cheap hosting if you ask right, free forums = bad if your serious.
