The current number of visitors wouldn't be a reason for me not to try it. If run well, the forum may easily increase the number of visits and page views per day. Some successful sites consist only of a forum with no additional static content. WebmasterWorld is a good example.
The main task you have is to create enough critical mass, both in number and quality of posts, and number of contributing members. Visitors do not automatically convert to contributers and you have to make it easy for them to do so. Seeding the forum yourself with meaningful threads helps. An easy sign-up procedure or posting without the need to register and enter silly questions and captchas also helps.
For me the reasons NOT to become active in a forum as a contributing member are:
No posts at all or a large number of empty sub-forums
A large number of stale threads where first-time visitors don't seem to get the attention they deserve to become returning contributers
No welcoming of new members
Large signatures or avatars which give me the impression that self-promotion of the members is more important than thread quality
Difficult sign-up procedure
Rude language, spam posts or other signs of missing appropriate moderation
The absolute number of users or threads in a forum is often not a reason for me not to join. The niche may just not have that many active contributors.
Well, I will say that the question you have asked is wrong, because until you open a forum no one is going to get an idea about your forum opening. Start with a few topics related to that niche.
Nowadays, social networking sites are in a boom, so try to create your community their by helping others and communicating with them create a group and add members to it. One you are popular in few popular sites, just spread a word on your community about your new forum and ask your members to join there. Put a section for your community discussions on your forum as well so that your social communities members will be in touch there as well.
One you get consistent members and discussions on the forum, you won't have to do much. All your members will do the rest of the things. If you have a good capital base try to purchase space on some popular sites to place your forum banners.