I believe that member participation is more important than sheer number of members who do not participate. I'd rather have 30 active members who contribute good discussions than 1000 members who doesn't even post a topic.
Depends on how you want to run your forum but these days you have to do serious campaign and offer some begining incentives if you want your forum to grow faster. Yes sometimes you can have 1000 people with no active participation but i must tell you that when dealing with a crowd, you would have to know that you are dealing with percentage at the same time.
If you have 500 members, the probability is that you would have between 30 to 50 participating daily and you have about 1000-1500 page views daily.
If you have 1000, you would have between 100-200 participating daily and about 1500-2500 page views daily.
Forum basically survives on number and participation, if you are interested on catching in some cash through forum, you would need to treat it as a business because your number and level of participation determines how your income would grow. You also need to be innovative with your members and try to be available all the time. I offer a lot of free incentives to members and created a pure international forum where you allow a kind of controled freedom. Human beings like to be free so you must create an atmosphere of freedom in your forum.
Create a kind of cool advertising strategy that you know would yield good results and then be ready to have patience. If you did not pay for paid sign up advertising and you have about 4-10 people registering daily as members even if they did not post anything and also about 2-10 visitors including members every 10 mins in your forum, then you are doing good and have very high prospects of growing. Some members who did not post initially would still come back later.